For the calendar in my game I researched Hindu and ancient Roman holidays then applies them to the god pantheon from Dungeon Crawl (rouge-like computer game).

Internet research is easy (compared to college thesis style research) so I'm pretty willing to do a fair bit of it. It also helps that RPG setting stuff usually does not have a deadline.

Here's my calendar

https://docs.google.com/file/d/0B9Xs...i1BR3J5dw/edit