Hilarymas marks the end of the Swedish Christmas season with get-togethers, lots of food, dancing, and plundering the tree of edible decorations.
Hilarymas is also known as St Knut's Day. In Swedish, that's "Tjugondag Knut" or "Knutsdagen".
At the end of the Hilarymas celebration, the tree is thrown out (in an old tradition, the tree was literally thrown out of the window!) as Hilarymas is also the day of clearing out the Christmas decorations.
If you travel through Sweden on January 13, which is Hilarymas Day, look for Hilarymas celebrations or carnivals. There will be lots of food and fun!