What's the best themed annual 'festival' (example in details) that you attend or have attended?

I was reading about the Rattlesnake Roundup in Sweetwater Texas and wondered what kind of experiences other people have had at themed festivals.

I am a (I thank my stars) transplant to the state of Illinois, and it seems as if the Testicle Festival is a state-wide tradition invented here.

I’d much rather eat a rattlesnake. personally.