I'm not really sure why there has to be "one" holiday at a time. I mean why would all the races of Norrath celebrate "common" holidays. Wouldn't it be nice to even mimic the real world to make good fantasy, perhaps the Kerran's would enjoy the Summer more than the winter and the Dwarves wouldn't even notice a season from below ground. Shouldn't worshippers of Brell have their own celebration? And even then you could have a holiday for certain worshippers within a town and not others, why can't the town have multiple holidays.
For my world I'd have Regional/National, Religious, Racial, and Cultural holidays... Some coincide some not. I mean it would be cool to see Druids doing a nightly ritual no matter where they are (perhaps the NPC's wouldn't be available for quests/merchant services). Maybe Kerran's wear custom paint during the summer solstice, or City of Freeport denizens celebrate Freeport Independence day. et al.