First Friday?

I have a question.

If one were to count backwards in seven day increments from today, how many increments would it take to find a day that was not called "Friday?"

I have done a little reading on the Wikipedia, Week, and see that there were interruptions in the sequence of continuous weeks (officially, at least) in France between 1973 and 1801 (they tried a 10-day week (oy! the French are so silly sometimes!), in the Soviet Union (they tried 5 and 6 day weeks from 1929-1940), and China and Japan (which have a specific time that they adopted the 7-day week, around 1000 AD).

This article indicates that the Jews had the 7-day week no later than the 6th centurty BC.

I'm curious.