Pokémon Go ‘Special Anniversary Pikachu Celebration’ event guide
via polygon.com
Short excerpt below. Read at the original source.
Pokémon Go’s “Special Anniversary Pikachu Celebration” update is bringing back most (if not nearly all) of the costumed Pikachu we’ve seen throughout the years. The event runs from July 13-20, but the spawns will shift every few days with new Pikachu rotating in and out.