I have to be honest, the replies I'm seeing for anyone running PhPBB are not very helpful.
I'm not a developer, an engineer sure, but that doesn't mean I can just figure out the best place to drop some code. The fact is that PhPBB is probably the most popular forum program out there and your product only works with it through a serious amount of work.
EasyMod is a 'modification' auto-installer for PhPBB that is incredibly popular. You basically write a text file with your code snipets and what it does is parse it and automaticaly drops your code in places throughout the template files, etc.
If you were to write an EasyMod file, you could have your calendar auto-deployed as both being able to drop in HTML and have it reside as a post, or have it display on the main page which is what everyone else does with their free calendar mods for PhPBB.