There are numerous threads out there on the subject. Search is your friend.
There are apps out there, one is called advanced task killer, that will allow you to shut down or kill things running in the background. Some think the calendar app is doing it, but my personal feeling is that it isn't just "one" thing thats doing it. I think for whatever reason, android isn't shutting something down like its supposed to and it keeps the phone awake. But that something could possibly be "anything".
You will have to see if there is an app or something you just installed or if its another process or widget of some type keeping the phone awake by trial and error using the task killer.
Personally I don't bother with the task killers anymore. Some swear by them, others don't. I think its becoming more of personal preference than tried and true fix now.