To usher in autumn nights in the Northern Hemisphere and spring days in the Southern Hemisphere, Niantic today announced the Pokémon GO September equinox in-game event. The Equinox event will run from 9:00 P.M. BST on Friday, September 22 to 9:00 P.M. BST on Monday, October 2, 2017. During the event players can earn double Stardust for catching Pokémon and hatching Eggs, collect special 2 km Eggs at PokéStops and Gyms, and gather special boxes from the in-game shop including the all-new Super Incubators, which hatch Eggs 1.5 times faster. Additionally, registering a new Pokémon to your Pokédex will earn you triple the normal XP during the event time frame.
You can read the official blog post on the Pokémon GO blog.