It's an impressive calendar. The appearance and functionality are great.
As with anything relying on Ajax, there are some small usability points. When entering a new event, be careful not to click outside the event screen; if you do that by mistake, as I did a couple times, the screen will close (just like a lightbox photo pop-up), and you'll lose the data you entered. Also, if you have a vertically short screen (I was using my laptop), you'll want to have the screen maximized, or you will have trouble getting to all the fields on the screen and saving.
I also noticed a common PHP foible - if you use an apostrophe in a field, when it's saved you'll have a slash along with the apostrophe. That's probably a one-line code change to fix, and I'll chalk it up to its newness.
All in all, very good stuff!