Using electrical tape, I split a whiteboard into sections. Voila, whiteboard calendar.
A teacher friend of mine already knew exactly how to do this--apparently it's common knowledge among teachers.
I did look if there were either large, disposable calendars (no) or existing whiteboard calendars (they're laminated posters that don't actually erase). A 3ft x 2ft (1m x 0.6m) whiteboard costs $30, and you can get electrical tape for $1.
The stickers look bad--I half-assed them. I need tons of big stickers or none at all. This was way too tentative. I do want some kind of decoration, though.