Overwatch 2021 is full of seasonal events, so here is a "calendar" with all the dates you need to know! Bear in mind this is what we know so far, and also based off the dates from previous years. Although, many sources have alleged that Overwatch is behind schedule in part due to the pandemic. However, the events that have happened (and most likely will) in chronological order are:
Overwatch 2021 Events Calendar: Dates to Know
Lunar Year Event
The first 2021 event was this year's Lunar Year Event which ran Feb. 4-25. The Lunar Year event featured the "Capture the Flag" game mode for the first time, and it has since returned whenever the Lunar Year event is live.
Overwatch Archives events bring new bits of lore from the past for players to experience in an immersive PvE mode that has, for the most part, been praised. Last year's started on March 12 and ended on April 2, and with Roadhog's events coming to a close it might just be right around the corner.
It is worth mentioning that Overwatch Archive events used to happen in April and not March pre-2020.
Ever since 2017, Overwatch has been celebrated its anniversary in late Spring. Last year, the event ended on May 21 so a good guess would be for the event to end sometime between May 19 or May 22.
The first Summer Games event took place on Aug. 2, 2016 and was the first kind of seasonal event that went live on Overwatch's servers. It introduced Lucioball back then, and has since brought the game mode back with both returning and new cosmetics for players to purchase. Expect it to start around the same time this year/
Dr. Junkenstein made his debut on Oct. 11 and Halloween Terror has since probably had the "closest" start dates. In two consecutive years it started on Oct. 10, so players can expect to play Junkensten's Revenge around the same time this year.
Mei's Snowball Offensive was introduced in this event, along with multiple cosmetics. The start dates are similar to Halloween Terror in that they have been punctual, ranging from Dec. 10 to Dec. 15. The event ends early January of the year that follows it, but players can expect the same timeframe this year.
Additionally, Blizzard runs mini-events throughout the year focused on specific heroes. The most recent one, the PachiMarchi Challenge, focuses on Roadhog. These events give players a chance to grind toward specific hero skins.