Billy - There is a Calendar hack to the phpBB software. And also a hack to that hack which adds in Event tracking.
Originally it was for birthdays, then modified to be driven by forum entries. I am not sure if it has the mini-calendar to go with it.
This is actually an add-on to phpBB, but I think you can glean the code a little easier and adapt it for you purposes this way.
The Calendar hack is at
http://www.phpbbhacks.com/viewhack.php?id=152. And the Events List hack for that is at
http://www.phpbbhacks.com/viewhack.php?id=1898.