https://www.gamespot.com/articles/last-day-for-pokemon-gos-christmas-event/1100-6455864/

The holidays may have come and gone, but Pokemon Go players still have one more chance to take part in the game's second annual Christmas event. The in-game event concludes tomorrow, January 4, giving you a few more hours to catch a rare festive Pikachu and take advantage of increased spawns for certain Pokemon.

The holiday event began last month, on December 22. Like the first Pokemon Go Christmas event, it introduced a Santa hat-wearing Pikachu, which can only be encountered during the event. This special Pikachu can evolve into a Santa hat Raichu, and unlike the previous holiday event, you also have a chance to hatch a rare Santa hat Pichu.