From Halopedia, the Halo wiki
Seasons are a feature of Halo: Reach, Halo 5: Guardians, Halo Wars 2, Halo: The Master Chief Collection, and Halo Infinite, primarily focused on their games' respective competitive modes. Seasons have typically accompanied new ranked settings, a handful of title updates and patches and accompanied the release of DLC. Leaderboards on Halo Waypoint are additionally organised by season, with a new season seeing the reset of leaderboards and new matchmaking playlists.
Seasons were introduced to Halo: The Master Chief Collection following the 2019 release of Halo: Reach and the collection for PC, serving as the game's ongoing post-launch support with each new season bringing new cosmetic customization unlocks and gameplay improvements.
Halo 5: Guardians
Halo Wars 2
Seasons are present in Halo Wars 2. During the first year of the game's life cycle, seasons were released roughly once a month to coincide with downloadable content releases or other events such as the launch of Xbox One X. Since then, the output of Seasons has slowed to as little as one per-year. Seasons are a fixture of the game's ranked multiplayer, used for leaderboard and rank resets. Season refreshes are typically accompanied by a Title Update, and may have several mid-season patches.
Halo: The Master Chief Collection
Seasons were first introduced to Halo: The Master Chief Collection with the addition of Halo: Reach to the collection in December 2019. Players can earn season points and spend them in any Season of their choice to claim its rewards. Initially, 343 Industries intended to support the game with seasonal content roughly every two months, planing to give the game ten total seasons. Later in Halo: The Master Chief Collection's lifespan, it was announced that Season 8 will be its last update on a seasonal model, however the game will still recieve more updates smaller that are in scope, with additional features, fixes and content, which are going to be released in 2022 as they're ready.
Season points are a new in-game currency introduced with the Season system. They can be used within the various unlock tiers of each season to unlock armor permutations, skins, nameplates, Firefight voices, and more. Initially, season points could only be earned by progressing in the game's rank scheme through regular multiplayer, though the addition of Challenges has resulted in a wider array of season point earning potential across multiplayer, campaign, Firefight, Forge and custom games. Season points are a purely in-game currency, and cannot be bought through real-world purchases. Season points are not reset between seasons, so any points earned will carry over into subsequent seasons.
Seasonal content for Halo Infinite multiplayer will include armor permutations. Four seasons have been mentioned for the game, though due to its pre-release nature these aspects are subject to change. Like The Master Chief Collection, seasons are expected to last and release every 2-3 months. Unlike The Master Chief Collection, Halo Infinite's Seasonal progress will be based on earning XP via the completion of daily and weekly challenges; each unlock across the 100-tier Pass will require the same amount of XP.