Nothing wrong with your code, in fact it works great, and looks great. Thanks and great job. I use prototype.js for about everything though. To be honest, the reason is ... well, because I always have. jQuery.js and prototype.js do not seem to play well together at all (this is my first time using both). As such, using prototype.js to handle the request to get your calendar, popped jQuery right in, and my code was pretty much done.
I also made a slight change so I could use my initial HTTPRequest(prototype remember) to pass it a 'field name' for it to include in the onclick event for a date. You likely already had a way to do that, but I couldn't figure it out in the time I have had with it thus far. I will of course study it some more and use the intended method when I figure it out.
It's a ruthless hack I know, but I liked your calendar and really wanted it. Thanks again.