The oddity results form the zeal of an archbishop of Bracara Augusta (modern Braga, in northern Porgual) during his efforts to convert the local population. In short, his understanding was that the original roman names for the days of the week were non-catholic as they were based in astrological "deities". As archbishop and apparently a man of great intellect and persuasive power, he managed to change the local tradition.

This happened before the formation of Portugal and at a time when portuguese as a language didn't exist. Instead, the language was more properly Galician-Portuguese, which would in time evolve into a family of languages, most notably Portuguese and Galician. In fact, in medieval texts, Galician also contains the same words "segunda-feira", "terça-feira", "quarta-feira" and so on. There are pockets in Galicia where they are still recognized by older people, although the Castilian forms have replaced them in most of the region. As the galician speaking community continues to recover their autonomy and push the differences, this could be changing again, and I found this little poster to illustrate that.

Back on how it happened, in a litte more detail. Bracara Augusta was a significant city in roman times and became the capital of a suevi kingdom. The suevi/suebi were a germanic tribe, which settled in the northwest of the Iberia Peninsula at the late days of the Roman Empire, in the 5th century. Over a century later, Saint Martin of Braga, at the time simply Martin, arrived from the East. He was born in Pannonia, where a suevi/suebi community was also established (but I have no idea if he was ethnically suevi or germannic, I hope someone more knowledgeable fills this is) and had traveled in the holy land before coming to the western end of Europe. He rose in the local christian ranks to Bishop of a small church and eventually to Archbishop of Bracara Augusta. In his religious zeal/endeavours, he replaced the pagan names of the weekdays with more "christian" ones.

When Portugal formed, six centuries after Saint Martin of Braga and a few of muslim rule, the terms were still in use. Portuguese incorporates a lot of arabic vocabulary, a latin base, some celtic and some germanic, but in this case, these words are an innovation of the language family.

An online source on Martin of Braga in Iberia: Follis, E. K. (1992, December 15). St. Martin of Braga : sources for his tolerance toward the rustici in sixth century Galicia (T). Retrieved from https://open.library.ubc.ca/cIRcle/collections/831/items/1.0086519