Talk:Snowdin/@comment-27524978-20161225030641

Considering the Christmas tree, wreaths, Papyrus mentioning Santa Claus and other holiday themed gifts around the town, it's safe to say that the underground timeline takes place in the month of December, at least before Christmas day.

Because of both Flowey and Frisk, however, the monsters are constantly stuck in December while the human world moves on. That would explain why we see monsters on the beach doing summer-related activities in the True Pacifist credits.