Knowing Polish and learning Russian had me do a couple of double-takes on days of the week. We call Sunday "niedziela" and the week "tydzień". I guess identifying Sunday with the week makes some sense, and one of the languages just switched it around at some point.

I always thought of "niedziela" as coming from "nie dzielić" / "не делить" / "no dividing/separating", though it doesn't really make sense as name for the day.