This article is for the festival that takes place in the world of Night in the Woods rather than the supplemental game of the same name.
Longest Night is a festival held in the world of Night in the Woods, and seems to be based on both the Winter Solstice and Christmas from our world. Longest Night is an ancient event, as even in the days of the Lost Constellation fable Mae's Grandfather recites to her, it was still common (and known as Longest Night even then). Although how Longest Night is celebrated is mostly unknown, one pastime was to watch and spot all thirteen of the Constellations, usually done around a campfire.
So far, Longest Night has been the setting for both supplemental games, Longest Night and Lost Constellation.