From 20:00 UTC on May 1 until May 14, 2018, Fighting-type Pokémon will spawn more frequently as part of a special battle event.
Apart from increased spawns, there will be double the bonus XP for Gym battles and Raid battles. Raid battles also guarantee 3,000 Stardust and at least one Rare Candy if players win.
Gyms will also have some extra bonuses, such as a 2x badge multiplier. Even just spinning the disc for the Gym stop will give out more items than usual.
Pokémon GO is currently available on iOS and Android.