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 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.
 * Seasons will be continually updated with more features and content.
 * Different new Mega-Evolved Pokémon will be appearing in raids each Season.
 * Certain Mega-Evolved Pokémon will 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 will expand from 10 to 24 ranks, and the top four ranks will have all-new names and corresponding badges. The expanded ranks will be accompanied by a totally new reward structure, which gives Trainers more opportunities to earn exclusive rewards through ranking up.
 * GO Battle League rewards will be tied to the theme of the current Season.
 * GO Battle League Seasons will occur in tandem with the overarching Seasons and will rotate formats on the same three-month schedule.

Trivia

 * This mechanic was announced in the update GO Beyond.