Week 10

In my other country I have been used to single dates - like March 5th , Nov 16th etc.

In Norway, at least at work, they talk and write about Week 1, Week 2 etc.
If the specific dates are noted on the lists using the Week number – no problem.
When it only says Week 10, I have to count the weeks from the beginning of the year – or find a calendar listing both.

When did this system of using weeks as time units develop in Norway, and why?
Did it come from the States?

Is it also used by some firms in my other country?