Hello, So I'm facing with the same issue of having 3 weeks in calendar instead of 4 and its really frustrating, with no fix provided by microsoft, but I found a workaround. Reason: Calendar size is small since notification center is occupying space on top of it which cannot be disabled. So, we're stuck with 3 weeks due to available size on the screen. workaround : Go to settings > system> display> scale and set it to any scale less than existing. I've set 125%. Now I can see all 4 weeks in the calendar. Before:
After:
Hope this helps.