I have a few that would all be for kicks and giggles;
1)(june) Graduation/ Prom Season for all the newly trained members of the horde/allaince(horde should ALWAYS be said before alliance)
2)(aug)late summer celebration where players have quest chains in beach areas like tanaris, stranglethorn etc. BGs could all have like a relaxed sunset colored theme just to make them a bit out of the ordinary and players could get a tan debuff that makes their skins a shade or two darker.
3)(could be any time of year that needs a random-ass holiday) Stormwind Founding/Orgrimmar Founding celebration where players learn about the events that occurred in said cities and can see visions of historic events (similar to how the arthas chain plays out in icecrown) relating to the cities. Also, during the ceremony,both cities could be sort of buffed up security wise (more 81 elites for example) and there be a extra special reward for killing Thrall or Varian during the event. Likewise, defending your city could give you a "Savior or Stormwind" "Defender of Orgimmar" title and hell, maybe even a unique mount. In order to make attacks/defends nice and organized, you can have a sort of ritual countdown where something special will happen just to give justification to a wintergrasp-like countdown where a raid/counter-raid could prepare. I think the event would work well for Blizz in November since a ton of games get released every year in that month and it could be a way to keep players in wow during that.