Runs from December 13 to January 1.
There are going to be 3 new champion skins: Ambitious Elf Jinx, Santa Draven, and Snow Fawn Poppy. Each one will set you back 1350 RP.
There are also Naughty or Nice mission variants that will be available during the Snowdown Showdown event. These missions will also have exclusive rewards such as emotes, limited-time poro skins. The website also teases "and more", but never specifies what that might be. Once you complete the “Be Naughty” or “Be Nice” mission, the other path and its rewards disappear. You’ll earn the Dravenbread emote for completing the Naughty missions and Braumbread emote for the Nice ones. Only matchmade games count for missions.
During the event, you will also be able to gift your friends a Santa Baron 2017 icon for 1,000 blue essence. While you cannot buy this icon for yourself, you can self-mystery gift champions or skins to yourself.