From Halopedia, the Halo wiki
An event in Halo Infinite is a special update with limited-time contents.
Events in Halo Infinite are around a theme. Some events have event passes, which are free Battle Passes that are active alongside any other owned Battle Passes for a limited time. Each event challenge that is completed advances the Battle Pass by one full level. As there are a limited number of event-specific challenges available while the event is active, completing the entire Battle Pass requires participating in multiple events over the course of a season. During the event, the Shop also features several bundles that are thematically related to the event.
List of events
Please see the respective event pages for their Event Passes.