Equinox

The Equinox is a seasonal event in Pokémon GO that begins in late March and September. This event celebrates the around the world.

Features
During this event, Trainers were rewarded with double Stardust for Catching Pokémon and hatching Eggs. Eggs had an increased chance of hatching Pokémon such as Chansey, Mareep, and Larvitar, that do not regularly hatch from these eggs or they do not do that so often.

Additionally, registering a new Pokémon to the Pokédex during this limited-time celebration awarded triple XP.

Pokémon Eggs hatchlings
Only these Pokémon were hatching from eggs during the event. After this time the all hatchlings were the same as prior the event.

Shop special offer
Special offer bundles were also available from the in-game Shop and featured items such as Lucky Eggs, Lure Modules, and Super Incubators which hatch Eggs 150% faster. This offer was available from September, 2017 through October , 2017.

2019 Event
The event will take place between March 19, 2019 and March 26, 2019.

Grass-type Pokémon—like Oddish, Exeggcute, Sunkern, and Shroomish—will appear more frequently in the wild. Lunatone and Solrock will switch hemispheres at the start of the event in celebration of the equinox. These Pokémon will stay in their new hemispheres after the event.

Two new moves will be permanently added to the game.
 * Acid Spray: Arbok, Victreebel, Tentacruel, Muk, Alolan Muk, Quagsire, Qwilfish, Octillery, and Swalot.
 * Leaf Tornado: Victreebel and Shiftry.

Raids
Grass-type Pokémon will be available to challenge in raids.

Field Research
Limited-time Grass-type Pokémon Field Research will be available during this event.

Trivia

 * The 2017 event was extended by 24-hours, following a login issue on September 26, 2017.
 * The event ended at exactly 1:00pm PDT.
 * This 2019 event was confirmed by dataminers network traffic almost two weeks prior to the announcement.