Snoggletog

The only known Viking holiday on Berk and a spin-off of "Christmas" shown in the 2011 holiday short film Gift of the Night Fury.

In Berk they celebrate Snoggletog during the winter time (what part of the winter is unknown, but by the time they're celebrating there is already a thick blanket of snow on the ground), and the Vikings decorate the village with lanterns (called "Snoggle Balls), shields, and other various weapons they aren't using anymore. The most notable decoration is the "tree" that they fashion out of green boards and shields--it's obviously modeled after a traditional Christmas tree.

Snoggletog feast in the Great Hall

During this time, the Vikings act very pleasantly and loving towards each other. They exchange gifts in stockings and each other's helmets and believe that Odin is the one who deliver's the presents found in the helmets. The holiday ends with a huge feast and party that's held in the Great Hall.

Snoggletog was apparently always about celebrating "Viking" pride, but in the current timeline of the franchise in Gift of the Night Fury, Snoggletog is also being used to show the unity between Vikings and Dragons.