Niantic has announced a special Water Festival for Pokémon GO that will take place from March 22nd, 2017 until March 29th, 2017. Full details were announced in their press release.

Make a Splash, Trainers!

Trainers, Get ready to make some waves in Pokémon GO! There’s a global Water Festival in progress, and Magikarp, Squirtle, Totodile, and friends are looking forward to celebrating with you! Starting today at 1 P.M. PDT, you’ll be more likely to encounter Magikarp, Squirtle, Totodile, and their Evolutions all around the world. Additionally, when you’re out exploring areas where Water-type Pokémon more commonly appear, you’ll have a greater chance to encounter several of the Water-type Pokémon originally discovered in the Johto region, and maybe... you might even encounter Lapras.

Also, starting later today, you’ll notice a new avatar wardrobe item—the Magikarp Hat!

The Pokémon GO Water Festival is a limited-time celebration and will come to an end on March 29, 2017, at 1 P.M. PDT, so get outside and GO catch some Water-type Pokémon!

Source: http://pokemongo.nianticlabs.com/en/post/waterfestival2017