a new feature in Pokémon GO starting December 2020 was the inclusion of seasons. These seasons are a combination of the meteorological temperate seasons as well as being themed towards other elements going around in Pokémon GO and the Pokémon franchise in general

List of Seasons

Season of MischiefSeason of MischiefSeptember 1st 2021 - December 1st 2021
Season of DiscoverySeason of DiscoveryJune 1st 2021 - September 1st 2021
Season of LegendsSeason of LegendsMarch 1st 2021 - June 1st 2021
Season of CelebrationSeason of CelebrationDecember 1st 2020 - February 28th 2021