The Nahua (Aztec) year was 365 days long, with 18 months that were 20 days long and an additional 5 "hollow" days which were considered a time of cosmic instability (a potentially dangerous and unlucky time). Leap years were accounted for by official calendar-keepers who would choose when to add the extra day; Michael E. Smith does briefly mention in his book 'The Aztecs' that there is some contention about how leap years were handled, but all was ultimately calibrated by the passing of the Pleiades through "the zenith of the midnight sky" every 52 years.

So, all that being said, the Aztec 'week' was five days long, with four days typically dedicated to work and the fifth day serving as a 'market day' in which people rested from their work and went to large markets to make purchases for the coming week. Or, if you were a vendor, you would take your produce from the preceding week to sell. The markets were open daily but many more vendors would be in operation on the "weekend"/market day.

The market of Tlatelolco was particularly famous for its size and extremely wide variety of goods. Jacques Soustelle's 'Daily Life of the Aztecs' states that daily attendance was 20,000-25,000, and every fifth day the great market was attended by 40,000-50,000 (I get a lot of joy out of the fact that on the weekend, the Aztecs also chilled out and went to the "mall", much like Americans in the 1980s/1990s).

Edit: I'd also like to add that the Tlatelolco market, in addition to having unprepared food available for purchase, also had a section for food stalls with prepared snacks and meals for sale. Cortes' conquistadors reported of one snack that was made of ice brought down from nearby mountains on the backs of runners, shaved and sweetened with honey and fruit juice. Tlatelolco had snowcones!! I love the Nahua. Sorry that this has nothing to do with the concept of a week (unless you count the fun of getting to eat a snowcone at the mall on the weekend).

For ancient Egypt, each week was 10 days long. The average person worked 9 days and the 10th day they were off.

Source: Dr Bob Brier's The History of Ancient Egypt lecture series. Available on Audible.

Romans it gets a bit more complicated. They had two major calendar systems, the one used before Julius Caesar created the Julian Calendar which is still in use by the Orthodox Church and only slightly differs from the Gregorian Calendar that we use today. The extra day in February every 4 years I believe is the big addition the Gregorian calendar made but I'm not 100% positive.

The Roman work week was pretty varied. Essentially the average Roman worked every day of the week outside of holidays and festivals. These were however pretty frequent occurrences and could last weeks. It varied per province if not per city depending on local customs and state holidays.

Source: SPQR by Mary Beard