[Update]
Niantic released a picture of the hat that Pikachu will be wearing during the celebration event for Pokémon Day. You can see an image of it below:
The original story is below.
Pokémon Go is celebrating Pokémon Day with a special wild encounter that is both cute and permanent for the Pokémon. Niantic will be holding this event next week starting on February 26th.
Every time a player encounters a wild Pikachu they will be wearing a “festive party hat”. Unfortunately, they did not provide a picture of this hate, but I can image it would be something cute. Any Pikachu you catch during this event will permanently keep its party hat.
This event is running from February 26th to March 6th, so try not to miss out on party hat Pikachu. Pokémon Day is actually on February 27th and is celebrating 21 years since the launch of the first Pokémon game back in 1996.
This is not the first festive hat that Pikachu has had. For Christmas, Pikachu could be evolved into a hat-wearing version of Raichu. This could be the case for this festive party hat, but we will have to wait to see. Hopefully we will get a photo of this hat soon.