The Pokémon GO team released the following statement on pokemongolive.com yesterday:
With cool autumn nights quickly approaching in the Northern Hemisphere and sunny spring days arriving in the Southern Hemisphere, we thought it would be the perfect time to celebrate the coming equinox with an in-game event. New seasons mean new adventures!
From 1 p.m. PDT on Sept. 22 to 1 p.m. PDT on Oct. 2, 2017, you’ll have the opportunity to earn double stardust for catching Pokémon and hatching eggs as you walk around your neighborhood discovering new sights. While you’re out exploring, be sure to visit your local Pokéstops and gyms to collect special 2-km eggs that have the opportunity to hatch Pokémon such as Chansey, Mareep, Larvitar and more. You’ll also be able to get special boxes from the in-game shop that feature items like lucky eggs, lure modules and all-new super incubators which hatch eggs 1.5 times quicker.
Registering a new Pokémon to your Pokédex during this limited-time celebration will earn you triple the normal XP. So that makes this a very good time to battle against the legendary Pokémon originally from the Johto region. Don’t forget that Raikou, Entei or Suicune will each travel to a new part of the world on Sept. 30, so make sure to battle the legendary Pokémon currently in your area while you can.