I don't know when it changed, but you cannot drag calendar events (assignments) into months other than the currently displayed month. You used to be able to use the small calendar at the top-right by changing it to the target month, then drag assignments from the main calendar into the target date in the small calendar. Now, these calendars will follow each other so they cannot be set to different months.
This problem is especially bad when months end on a Sunday, because you cannot even see part of the next month. When you get to see part of the other month, you can drag events onto a placeholder Sunday from next month, change the month, then drag them where they belong (this is also not a good solution).
Manually opening, changing dates, and saving for hundreds of assignments that changed months is very time consuming.
