Christmas hands down!
The cost of new decorations, regardless if bought before or after the holiday. The food, baking supplies, new linens and table center pieces. New outfits, hair cuts, holiday pictures, gifts, outing trips. Christmas cards and postage stamps. The cost of GAS while out buying said mentions. The electric bill from all the Christmas lights and electric powered ecorations being use. For those of us with kids, the multiple batteries that are required but sold seperately.
I think Thansgiving is a close second with those who travel to be family and or to attend those Thanksgiving Day football games and parades. And of course all the food!