*You could double the drop rates or some other rate on ever holiday...
Thanks Giving - Have a scavinger hunt... Who ever wins (Maby like the first 3 or 4) get an OBB and an OPB. (It would not make since to give them Hero Points) Also you might want to have a PvP event on this day.
Christmas - Have a snowy Prontera (Duh) and have a random drawing of presents... Have lots of the drawings turn out to be OBBs, some OPBs and then a few items... Nothing too good though. Also have some event like, Protect the town of Lutie from monsters that have invaded!
Valintines Day - I dont know, too far off... But be creative! =)
These are just some thoughts of mine...