The Gregorian division in 12 months and 7 days per week is quite archaic anyhow and has it's origins in the number of planets in the night sky that are visible with the naked eye, plus the moon and the sun. Ancient lunar/lunisolar calendars like the Babylonian calendar adopted this number, probably for religious reasons. (Even though one 7-day week roughly equals a quarter of a lunation, each lunation being roughly 29.5 days, a division into 6-day or 5-day weeks would have worked just as well. So there's more to the 7-day week then it being a division of the lunar cycle.) Even though the Gregorian calendar is a tropical solar calendar the ancient 7-day week stayed as well as the lunar 12 moon cycle and the lunar hollow/full alternation between months, as it did in the Julian calendar. But it was more a cultural thing then a mathematical necessity. All months and weeks really do is divide the days between solar equinoxes into blocks but this could have been done in plenty more ways mathematically. But.. Whatever. Elder Scrolls has a similar length year, with similar months and weeks.
Even though it wouldn't have been that difficult to make up a fantasy calendar that differentiates from that they probably opted not to do so because the calendar as it is is easier to relate to for the player. And I don't realy care enough about it to call it lack of lore depth or whatever.
Semantics: the Elder Scrolls calendar isn't exactly the same. The gregorian calendar uses leap-years, compensating for the fact that each year (1 solar orbit) is actually 365 days 5 hours 49 minutes 12 seconds long. The TES calendar (in Oblivion ans Skyrim that is) doesn't use leap-years and is exactly 365 days long.
And who knows whether it's a lunar, lunisolar, solar or celestial calendar??
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Only some of Breton society is based around the French, the more mountainous areas are more celtic based. 'Madanach' being an example of a non-French Breton name.
There are actually Bretons in France, they live in Bretagne. Their ancient Breton origins are Celtic. Over the ages they've become more and more Frenchified.
Elder Scrolls Bretons are based on those Bretons.