I think "There are seven days in the week" But "the seven days of the week" or "the day of the week". As a noun phrase "of". As a statement with no articles, just like describing location for other things, "in"

If you want to say "seven days" with week, you have to say seven days in the week. Not "of" the week. Melody's answer is good to illustrate an exception. "How do you say the days of the week in XYZ language?"

I think that depends on the context. There are seven days in a week, but I could also say that I am learning how to say the days of the week in Arabic.

What do you think? Any ideas at all?