From Diablo 4 Wiki

Diablo 4 is a live service game that will receive many updates throughout its lifespan and these will updates will include Events as part of each Diablo 4 Season.

Events During Seasons

Live events will challenge players to tackle specific challenges as they happen in the open world.

Sanctuary is a living world filled with people, creatures, and factions striving to meet their own ends. Attentive players should be on the lookout for new live events that will crop up each season. An example of a live event might be the warning of an impending invasion of the Drowned, which may last a weekend, or the arrival of a strange peddler amidst the crags of the Dry Steppes. These events provide gateways to new adventures and unique rewards.

Event Interface

Diablo 4 Events Interface (Beta)