When you start using your calendar in Outlook, the weekly numbering is disabled. If you want to activate it, do it yourself via the calendar options.

1. Click the File tab.
   
2. In the left corner, click Options.
   
3. Click Calendar.
4. Scroll down to Display Options, and select the Show Weekly numbers in the mouth view and in the Date Navigator.
5. Click OK
   
6. You now have weekly numbers on your caledar in Outlook.