They normally happen around major holidays, like Halloween, Christmas/New Year, and Easter. Other times the gods might throw mini ones at random. So, basically they'll take place whenever Michael and Auriea decide to have one, which means no one knows for sure when one will take place. However though, the gods will most likely give a heads up of when a major one will take place =]
I'm excited for too. I have yet to see one for myself |D

I hope I don't miss out on them =)