I won't lie: I've resubbed to WoW for Brewfest. No joke. I don't even drink in real life, but I love Azeroth's in-game event. The fun toys, the celebration, getting folks together to do things in game that aren't just for progression but for fun ...
Sure, some events get revamped, but there are some dead spots on the calendar. Wouldn't it be cool to have St. Patrick's Day in March, to ease the lull between the Love is in the Air and Noblegarden events? Make it some sort of Goblin celebration (neutral Goblins, not Horde). The real-world counterpart's largely a secular holiday these days, so all we need is some green and drinking. Perhaps a celebration of some Goblin businessman finding enough money to start the Steamwheedle Cartel?. Heck, maybe even make it so we can actually visit a small portion of the Undermine until Blizz wants to fully flesh it out; make it a holiday dungeon where we save Undermine from something Deathwing woke up.
Or maybe something new altogether ... August is kind of a dead month, and since summers are slow anyway, why not spice things up? Perhaps an Azerothian Scholars holiday honoring prominent members of the WoW community, from addon creators and theorycrafters to people who try to help make the official WoW forums useful. Other games do bonus XP weekends, so WoW could have something like that, where honor, justice points, loot drops and so on could be increased for a few days and load screens have the names of players who've stood out in the community.
What kind of holiday would you like to see added to WoW? Explorer's Day? Teacher Appreciation Day? Tigule and Foror Ice Cream Day?