Halloween is the most popular. Perhaps it's because MMOs are already into the spirit of the season, with characters dressing up, "trick or treating" mobs for their candy, and tackling spooky, scary areas where doom is sure to lurk. Or perhaps developers just really like pumpkins and haunted houses. Who doesn't?
With most MMOs sporting Halloween content, it's a safe bet that there's something for you in your favorite game. So hop on the hayride and get ready for a tour through the most terrifying, cute, and boo-tastic Halloween events in town!
[Update: Added more festivals... for justice!]