It's Christmas day! But since you and your two siblings, Dipper and Mabel Pines, the twins, are in Gravity Falls, something weird happens, but Christmas themed, so Rudolph or something could come out of nowhere or Santa or some weird creature, who knows

Dipper: He's wearing a red sweater with a Christmas tree on the front, and he's tied up in Christmas lights

Mabel: She's wearing a purple sweater with Rudolph's face on the front Hehe