Next week marks the 21st anniversary of the Pokemon franchise. To celebrate, Niantic is holding a small event to commemorate "Pokemon Day." From February 26 until March 6, every Pikachu in the game will be wearing a festive party hat. There's a photo below for anyone interested in what they look like.
Players will be able to keep this special Pikachu forever. There was no mention of whether or not Raichu or Pichu will have hats, but I would assume at least Raichu does (since the Christmas variation did).
Pokemon Day is on February 27, which is the date that Red and Green were released in Japan.
Pikachu Parties in Pokémon GO! [Pokemon]