It was announced on the official Pokémon Go website that:
Did you know February 27 is Pokémon Day? We’ve teamed up with our friends at The Pokémon Company International for their annual celebration of all things Pokémon, and we have a special treat for you this Pokémon Day! From February 26 at 1:00 P.M. PST to March 6 at 1:00 P.M. PST, special Pikachu appearing all over the world will be celebrating Pokémon Day by wearing festive party hats. These special Pikachu will keep their party hats forever, but they can only be found during this limited-time celebration, so make sure to keep an eye out for them as you explore!
The special Pikachu looks like the following:
If I evolve my special Pikachu, do I also get a special Raichu with a hat?