While you're waiting for an answer about the peculiarities of English, you may be interested to read this long answer about the origins of the seven day week, and why specific days are associated with specific gods/planets in the first place:

• When was it decided which day of the week it was, in order for today to be Wednesday?

It's a sordid tale involving market cycles, Babylonian astrology, Egyptian time-keeping, late Hellenistic fortune telling, and the monotheistic idea of setting apart a day for God.

Just for the sake of clarity, are we speaking about English/Germanic days of the week?

Because French, Italian, Spanish and Portuguese all retained their Roman roots. In addition, Greek retained the late Roman name for Sunday - "Dominicus" (The Lord's Day.)

Modern Greek calls the day "Kyriake" (Lord's Day)

I think a better way to phrase your question is “why don’t the English call it Lyeday?”

In your post, you break it up between Latin (Saturday and Sunday), Anglo-Saxon (Monday), and Norse (Tuesday through Friday). However, as you learn from the Interpretatio Germanica, these are all Proto-Germanic calques of the Latin names for the days of the week, which were borrowed from Greek. The German tribes and the Romans borrowed different words from each other, especially during the Migration period, and it would make sense for the German tribes to borrow well established systems and concepts like a set week. So, from this, all of the days of the week are a borrowing, but they aren’t the Romance names.

Our Tuesday through Friday are not from Norse (better named Northern Germanic), but are from the native Anglo-Saxon (or Old English) names for the days. It is important to note these were the Old English names for their gods, not the Norse gods; they came from the same traditions, but these were the folk’s religion. However, since we don’t have neat little written accounts of the Anglo-Saxan religion like we have for the Norse religion, it is an easy mistake to make. So, the Anglo-Saxon names were:

• sunnandæg (Sun’s day)
• monandæg (Moon’s day)
• tiwesdæg (Tiw’s day)
• wodnesdæg (Woden’s day)
• þunresdæg (Thunor’s day)
• frigedæg (Frigg’s day)
• sæterndæg (Saturn’s day) or sunnanæfen (Saturn’s Evening)

Now, the Germanic languages share these lists expect for Saturday. Saturday was something it seemed that nobody was sure what to do with. Old High German adopted, from Greek through Gothic, “Sabbath day” or Samstag in Standard German. Unlike most of the other days from Latin, I don’t believe any Romance language kept “Saturn’s Day,” but most switched to “Sabbath day.” So, they may have just switched like everybody else, but we can’t be sure. Not all German dialects (or languages, completely different subject) use this word; others use Sonnabend, coming from sunnanæfen, from above.

The other West German language (English, Dutch, etc.) presumably just kept Saturday because there was no clear comparison in the Germanic religion, and for whatever reason didn’t follow the trend towards “Sabbath day.” Although the Greek, Latin, and Germanic religions probably came from a similar source, or at least influenced each other, it is hard to line things up as different gods gain or lose influence in different pantheons (the Anglo-Saxon Tiw, the Latin Deus and Iuppiter, and Greek Zeus all come from the same word, but the god Tiw had lost influence to Woden and Thunor, compared to Jupiter and Zeus).

Finally, for whatever reason, the Northern German languages did not use this Saturn stand-in. Instead, they used laugardagr, which translate to “Washing day.” I want to say it goes back to some sort of tradition, or something similar, but that’s beyond my understanding. If English had followed this route, it would come out to something like “Lye-day,” as in the main ingredient in soap, lye.

So, why do we use the words we use for the days of the week? Unfortunately, the linguistics answer is just that we do. We say “Saturday” because that is what we call it. Language is a messy business; it is constantly changing, and most of the time, all we can do is describe what happen and not why.

Edit: I feel it important to note, that since writing this, I can stop signing "Lyeday, Lyeday, gotta wash up on Lyeday," a la Rebecca Black. Thanks OP...

In general, the days are named after Old English gods, which in turn derive from Proto-Germanic deities. It is only in one case (Thursday) that the name appears to have been influenced by the Norse form.

• "Tuesday" comes from OE Tīw from Proto-Germanic *Tīwaz, not Old Norse Týr
• "Wednesday" comes from OE Wōden from Proto-Germanic *Wōđanaz, not Old Norse Óðinn
• "Thursday" appears to been influenced by Old Norse Þórsdagr; the Old English form was Þunresdæg.
• "Friday" comes from hypothetical Old English *Frīg from Proto-Germanic *Frijjō, rather than from Old Norse Frigg.

It should be noted that these names were all calqued from Latin according to the so-called interpretatio germanica. The Latin names of the days were:

• Tuesday = dies Martis (day of Mars -- identified with Germanic *Tīwaz)
• Wednesday = Mercurii dies (day of Mercury -- identified with Germanic *Wōđanaz)
• Thursday = dies Iovis (day of Jupiter -- identified with Germanic *Þunraz)
• Friday = dies Veneris (day of Venus -- identified with Germanic *Frijjō)
• Saturday = Saturni dies (day of Saturn)

For some reason no Germanic equivalent was found for Saturn: that is why Saturday is the only day of the week that preserves the name of its original Latin deity/planet.

So the crux of your question is: why was no Germanic equivalent found for Saturn, when such equivalents were available for all the other deities/heavenly bodies? As far as I know, no one has come up with a definitive answer to this question. Tacitus, in his Germania, describes the religion of the Germans thus:

Of all the Gods, Mercury is he whom they worship most. To him on certain stated days it is lawful to offer even human victims. Hercules and Mars they appease with beasts usually allowed for sacrifice. Some of the Suevians make likewise immolations to Isis.

While the list of deities here does not exactly match those of the days of the week, it may perhaps be of interest the Tacitus describes three gods and one goddess -- exactly the same number as appear in the interpretatio germanica.

The spread of the seven-day week throughout the world can be traced to the Roman Empire, who adopted it from the Jews in Egypt in the first century B.C.E. The Jews themselves had been using it since before the tenth century B.C.E. Whether the seven-day week (with one day of rest) is of Jewish origin is disputed--while the six days of work followed by a day of rest is clearly attested in Jewish liturgy, Zerubavel (1989) points out that seven-day periods were documented by their Assyrian and Babylonian neighbors as being of import, while others around the world subdivided the lunar month more naturally into 5-, 6-, or 10-day periods. The Romans themselves initially used an eight-day week, nundinum, which has an etymological origin in the words for "nine" and "days," but that's another story.

In any case, we see in the early Empire the first evidence of adoption of the seven-day calendar and it becomes quite popular in the ensuing centuries. Certainly, it was already popular when made official by Constantine in the third century C.E. when he declared Sunday, dies Solis to be a holiday.

The Roman 7-day week was as following:

• dies Solis, "day of the Sun"
• dies Lunae, "day of the Moon"
• dies Martis, "day of Mars"
• dies Mercurii, "day of Mercury"
• dies Iovis, "day of Jupiter"
• dies Veneris, "day of Venus"
• dies Saturni, "day of Saturn"

Those of you who speak Romance languages (other than Portuguese) can already see the clear etymological relationship between the Latin names and names in modern Romance languages.

The Roman week was spread throughout its empire prior to the adoption of Christianity, and then farther by the Christians, and then by Muslims. As such, the majority of the names of the week in modern languages are either of pre-Christian origin, of Christian origin, or of Muslim origin (Falk 2009). The days of the week as ultimately adopted by English are all likely of pre-Christian origin, each corresponding to a Roman day of worship, but translated for local pagan consumption in a process known as Interpretatio Germanica (as you reference in your edit).

In places populated by Germanic peoples, when adopting the seven-day calendar, the Roman gods were substituted for local gods. Interestingly, this may have happened via early Christian missionaries in an effort to appease the local peoples they sought to convert. In any case, each of these gods had a lot of different names at different times and in different cultures, so the list of replacements below is very much a simplification:

• Sunnǭ, Sunna, Sól, Sunne, instead of Sol Invictus (Sun)
• Mēnô, Mōna, Māno, Máni, instead of Luna (Moon)
• Tīwaz, Tīw, Týr, instead of Mars (War)
• Wōdanaz, Wōden, Wuotan, Óðinn instead of Mercury (Poetry)
• Þūraz, Þunor, Donar, Þórr, instead of Jupiter (Thunder)
• Frijjō, Frīg, Frīja, Frigg, instead of Venus (Love)

So in Old English, we end up with:

• Sunnandæg, now Sunday
• Mōnandæg, now Monday
• Tīwesdæg, now Tuesday
• Wōdnesdæg, now Wednesday
• Þūnresdæg, now Thursday
• Frīgedæg, now Friday
• Sæturnesdæg, now Saturday

You may see the odd one out here, the subject of your question: Sæturnesdæg, literally "Saturn's Day". Why is Saturday the only day that was not "localized" via interpretatio germanica, instead taking its name directly from Latin?

We see various names for Saturday across Europe, none of which seem to derive from the name of a local god: in Scandinavia, modern Norwegian (and Danish) Lørdag and Swedish Lördag are supposedly from old Norse laugardagr, "washing-day," which falls suspiciously before the Christian day of prayer, Sunday.

Samstag is German and derives from Hebrew Shabbat. Parts of Germany call Saturday Sonnabend, referencing Saturday's place before Sunday ("sun-eve"), though this is only attested in the late medieval period.

And many, like English, seem to be adaptations of Dies Saturni: Frisian Saterdei, Dutch Zaterdag, and even non-Germanic languages like Irish "De-Sathairn," Welsh "Dydd Sadwrn," and Albanian "Shtunë."

Unfortunately, there isn't a consensus on why Saturday didn't end up getting a Germanic replacement for the Roman god. It is thought, though, that there wasn't a god in the old Germanic pantheon that quite corresponded with Saturn's function, or at least not one that merited worship. Some have proposed that Dano-Norwegian/Swedish Lørdag/Lördag actually has its origins in a corruption of "Loki's day," but this is questionable at best--there really isn't evidence of widespread, institutionalized worship of Loki.

It's not that there existed no equivalent for Saturn anywhere in the world, though. As a matter of fact, the naming conventions for the seven-day week made it as far as India, with the names of each day translated not by matching gods, but celestial objects. In the case of India, Saturn was changed to Shani, the Sanskrit name for the planet Saturn. Many south- and southeast Asian names for Saturday derive from the Sanskrit (see Hindi Shanivaar).

Indeed, this system spread as far East Asia. Days of the week in Classical Chinese were also derivative of the Roman naming convention: Sunday was 日曜日, "sun day," and Monday was 月曜日, "moon day." Tuesday throughSaturday were named after Chinese astronomical objects (Sun, Moon, and five visible planets), with which the Chinese associated with the elements: Fire, Water, Wood, Metal, and Earth. The order of the elements and that of the astronomical objects to which they corresponded mirrored the Roman ordering. As an example, Saturday was 土曜日, "earth day," earth being the element that was associated with the planet Saturn. While modern Chinese has abandoned this system in favor of simply numbering the astronomical objects (the days now roughly translated as "sun star," "first star," "second star," and so forth), those who speak Japanese will recognize that the Classical Chinese days of the week survive in modern Japanese to this day.

Before I get into actually answering the question, I would like to point a couple things out:

Sunday stems from the Latin, "dies solis."
Monday stems from the Anglo-Saxon, "monandaeg"

"Sunday" and "Monday" have parallel origins, both being calques from Latin.
dies lunis ~> monandæȝ -> Monday
dies solis ~> sunnandæȝ -> Sunday (see below for more on this one though)

Tuesday, Wednesday, Thursday and Friday are named after the Norse gods Tyr, Odin, Thor and Frigg respectively.

Norse diverged from English before or around when these names came about, and regardless English isn't descended from Norse, it evolved in parallel. The English words are therefore not from the names of the Norse gods, but rather from the Anglo-Saxon ones. Of course, they are largely the same figures under different names since both are branches of Germanic paganism, but really the only reason we associate the names with the Norse gods now is that Norse mythology happened to be better preserved in writing. The days of the week are thus more accurately named after Tīw, Wōden, Þunor, and Friȝe.

As for the actual question:

The Latin days of the week in imperial Rome were named after the planets, which in turn were named after gods [...] In most cases the Germanic names have substituted for the Roman god's name that of a comparable one from the Germanic pantheon.

Meaning that since heavenly bodies after which the days were named were in turn named for gods in Rome, analogous names were chosen in England. The Germanic pantheon didn't really have a deity corresponding to the moon though, so they just used their name for the object. Saturn is, as a planet, referred to in Old English by its Latin name, so that may be seen as a parallel origin to the use of "mona" for the moon. The Anglo-Saxons did have a "head god" like Saturn was for the Romans, but, seeing as Wōden has a different day named after him, that could get confusing (same goes for Þunor, the lightning god). As for the Sun, that's an interesting case, as there seems to have been a deity corresponding to the sun, but with the same name as the star ("sunne"), so again it follows the Roman paradigm, but you can't tell.

Source: Oxford English Dictionary
Edit: corrected "Frige" to "Friȝe"
Edit 2: grammar and clarity

We have to actually go father back then the greeks/romans for where the names and day of the week come from. The first instance of a 7 day calendar was from Babylon (though this was most likely a refinement of an earlier Sumerian calendar), and in the Babylon calendar every 7th day was considered "holy-day" which was given over to the worship of different gods based on the exact 7th day it was in the season. As an aside this is most likely where/when the Jews 'shabbat/sabbath' originated from, as Jewish calendar after this period only numbered Sunday through Friday but named Saturday. The Babylonians (possible even the Sumerians) started the naming of parts of the calendar after gods, though in their case it was the month and not the days. Skipping a lot of other calendars around during this to focus on answering where the names came from. The Greeks were the first to really start naming the days of the week instead of just numbering them, and as their calendar more or less came from the Babylonians (this is the part I am skipping), they named the days after the planets and thus their gods. Their week was: Sun(helios), Moon(Selene), Ares, Hermes, Jupiter, Aphrodite, and Kronos (sun -> sat).

Rome gradually replaced their nundinal cycle calendar with the 7 day week (earliest recording of the 7 day week Roman calendar is from graffiti in Pompeii from around 60 AD), by this time they had already changed the names of the week to reflect their gods. Their week was: Sun(Sol), Moon(Luna), Mars, Mercurius, Jupiter, Venus, and Saturn (sun -> sat). (Though it was until much later that the 7 day calendar completely replaced the nundinal cycle one in the empire)

Around 200 AD but before 500 AD the Germanic tribes were introduced to the Roman calendar (7 day version), and started using it as their own, but as they say "do as the Romans do" they change the names of the day to match their deities, mostly. Thus their week became: Sun(Sunna), Moon(Máni), Tiwaz(Tiw/tyr), Woodanaz(Woden/odin), Thunder day(Thor, it is actually 'Þunraz ' but that more or less translate to 'thunder'), Frijjo(Frigg), and laugardagr(litterly 'bath day'). (sun -> sat)

Now its important to remember that old English is mostly a Germanic language and starting in the 7th(ish) century old English(proto-germanic) is when started to incorporate Latin into its vocabulary. Why is this important, well simply, by the time Latin started to influence the core of the English language, we at that time we already had 6 good name for the days of the week and 1 throw away. And as people decided that 'bath day' was a little too silly for them, they did what the English language does best steal better words from other languages! The original name for the days of the week in old English are Sunnandaeg (meaning 'sun's day'), Monandaeg (meaning 'Moon's day'), Tiwesdaeg (meaning 'Tiw's day', Tiw AKA Tyr), Wodnesdaeg (Odin's day), Þūnresdæg (meaning 'Thunder's day'), Frigedaeg (meaning 'Frige's day', Frige AKA Frigg), and Saeturnesdaeg (meaning Saturn's day). (sun -> sat).

As others have commented, Latin based languages have retained their original Latin names. One thing to note about them though is the 'Sun' day in early Roman time was 'Dies Solis(sol)' but was later changed to 'the Lord's day' or 'Dies Dominica.'

--Most of this was taken from "Mapping Time, the Calendar and History" by EG Richards.

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