Niantic announced the next large Pokémon GO event which will be held from May 18 (US/EU) and May 19 (Asia/Japan) until May 25, 2017. During this period you can get rare pokemons more frequently.
To help you stock up for your upcoming adventure, more items will be awarded from every PokéStop you visit, and Poké Balls will be 50% off in the in-game shop. Your Buddy Pokémon is also in the adventuring spirit and will find Candies four times as fast for the duration of the celebration.
Whether you’re exploring your neighborhood or an exotic new location, make sure to watch for Rock-type Pokémon such as Omanyte, Kabuto, and their Evolutions. Keep an eye out—maybe you’ll even spot Aerodactyl!