Will evolving the Pokémon Day 2017 festive Pikachu give me a festive Raichu?
From this website it appears that you can:
You can't catch a Party Hat Raichu the same way you catch holiday Pikachu, but you can evolve it from a Party Hat Pikachu. That's as long as you have 50 Pikachu Candy you're willing to spend on it.
Image Source
As I commented, they did something similar for Christmas, which when you evolved that Pikachu, you got a Raichu with a Christmas Hat. It makes sense for them to keep it consistent between the events. However for the Christmas Event, you were able to capture a Raichu with a hat as well.