I'm pretty sure this is repeated around the world, but I thought one more blog post throwing it's weight behind it wouldn't hurt. Developers, do not forget that Visualforce ActionSupport events ARE case sensitive.
That easily cost me half an hour of visualforce debugging, and not for the first time!
Now, as a quick test to make sure you were paying attention, why won't my daysbox element rerender in this code snippet?? Use your eyes, and your minds..