I guess Labor Day and Memorial Day are the two biggest ones -- maybe all of the NPCs could take a day off, there could be giant barbeques set up in the capital cities, or players could take a day to remember those fallen in the past wars. Or you could go another route and just make up your own holiday: Grumpyfool of Azjol-Nerub recommends Gnome Appreciation Week (a whole week?), where Azeroth's shortest denizens can get some love of their own. A more recently based holiday might be fun, too -- maybe once a year we could remember Arthas' (it was Arthas, right?) zombie attack on our homeland.
I doubt this is going to be used to determine actual Blizzard policy. Seems more like just a fun thought experiment. But there are definitely some interesting ideas in there -- if Blizzard ever wants to add yet another holiday to the calendar, there are plenty of ideas.