Week numbers are used a lot in my experience. Features are often promised by week X. Events, work schedules and even vacations are also often referenced by week.
It could vary by country as well, when I grew up and went to school it was used a whole lot. I don't remember the last time I saw it though, and since moving abroad I definitely haven't seen it.
It varies a lot by trade and profession. Any place where complex scheduling is part of the job tends to use them (e.g., schools, manufacturing plants, accounting).