Season

Season is an in-game phenomenon that mimics the real world. Pokémon GO rotates through winter, spring, summer and autumn every three months and brings major changes:
 * Different Pokémon will be appearing more often in the wild, hatching from Eggs, and appearing in raids. Some Pokémon will be more difficult to find depending on the Season.
 * Pokémon appearing in the wild each Season are based on which hemisphere. For example, in the first Season, Pokémon related to winter will be appearing in the wild in the northern hemisphere, while Burmy, Darumaka, and other Pokémon related to summer will be appearing in the wild in the southern hemisphere.
 * Different forms of Deerling will be appearing in the wild depending on which hemisphere.
 * In-game events will be themed based on the Season they occur in.
 * Different new Mega-Evolved Pokémon will be appearing in raids each Season.
 * Certain Mega-Evolved Pokémon may have limited periods where they will have increased total CP and Trainers will have more opportunities to use these Mega-Evolved Pokémon more easily.
 * The GO Battle League has expanded from 10 to 24 ranks, and the top four ranks have all-new names and corresponding badges. The expanded ranks are accompanied by a totally new reward structure, which gives Trainers more opportunities to earn exclusive rewards through ranking up.
 * GO Battle League rewards are tied to the theme of the current Season.
 * GO Battle League Seasons occur in tandem with the overarching Seasons and rotate formats on the same three-month schedule.

Trivia

 * This mechanic was announced in the update GO Beyond.