Do you need some kind of special license to do it, or do you just put the name of the week in your title and hope for the best?

It's not an official thing, just make a submission with the week name in the title and if others make submissions for it, it will seem official.

Anyone can just start their own so there is no official theme week. The important thing is to promote it.

The second one. Just go for it.

(Sometimes people submit memes promoting it in advance too.)