One of the weird ones is our (English) weekday names: Sunday, Monday, etc. Those are apparently from German, but after their contact with the Romans, and after the Romans adopted the 7-day week. Which means it is after the Christianization of Rome, but before the Christianization of the Germanic tribes. They took the Roman week and put their gods' names in place instead.
It has to be said also that Roman calendars were actually "pillaged" from Greek culture and then tweaked until right - a pragmatic approach they followed in so many different cultural and scientific areas when interacting with foreign civilisations. Unfortunately, they didn't go as far as trying to align the 7-day week (of Middle-Eastern origin) with their Greek-ish months, and their legacy ended up being managed by a Church that valued immutability over everything else. So here we are, 1600 years later, still celebrating the months of Julius (Caesar) and Augustus when the weather is the best around the Mediterranean Sea, the month of war-god Mars in the best period to start military campaigns in the same region, and with four months named by numerical positions they've long since changed... and still completely mismatched with 7-day weeks.