A party celebrating cold weather, outside in the cold. Traditionally, white russians and hot toddies are served. Alternatively, whiskey. You can only go inside to use the bathroom, depending on host preference, and the more guests complain about being cold, the more successful the party is considered.
This type of gathering originated in the Southern Hemisphere, where warm blooded mammals paid tribute to the gods for the brief ability to breath and sleep and go without blistering in the sun.
After reading the farmers almanac, Sandy from Georgia bought a bunch of mittens at the bogo, anticipating hosting a cold weather party.