Yesterday, March 6, was your last chance to find and catch special Pikachu wearing festive party hats in Pokémon GO. They were made available in the popular mobile game for a limited time as a way to celebrate Pokémon Day 2017. The Pikachu will keep their party hats forever, even after evolving into Raichu. Pokémon GO also received its latest update yesterday, implementing some minor text fixes.
Party hat Pikachu can no longer be found in Pokémon GO
Pokémon Blog 