This is the biggest one, though some fandoms have their own. Sign ups are in the spring sometime and you get a few months to write a story.

If you aren't sure you want to jump in without trying a remix first, remix a friends fic - ask if they mind and then try a hand at it. It's a lot of fun - it's a great challenge.

The main rules of remix are - the story has to be complete, cannot be the one safe story you are allowed to mix, and you have to keep pairing/plot the same. Everything else can change.