Halo Universe Events
- Main article: Iris
- June 19: AdjutantReflex was taken over by another entity.
- August: Dr. Jonas Volman, the author of The Castaway Theory, returned from his absence, having gone missing in November 2005.
Real World Events
- May 11: The Halo 3 Beta was first released to a limited audience.
- May 16: The Halo 3 Beta became accessible to the public.
- May 31: Halo 2 Vista was released for the PC.
- June 10: The Halo 3 Beta came to a close.
- June 14: Halo 3: The Cradle of Life was found by a Bungie.net user, crazyBoy3.
- July 10:
- August 1: Marathon 2: Durandal was released on Xbox Live Arcade.
- August 10: An internal build of Halo 3 titled Halo 3 Epsilon was released to Microsoft employees.
- August 22: The first issue of the Halo: Uprising series of comics was released.
- August 28: Halo: Combat, the second part of the Halo: Landfall series, was released.
- August 29:
- Halo 3 goes gold.
- Halo 3 Epsilon was closed.
- The Believe Halo 3 marketing campaign launches.
- September 22: Hunted first aired on TV.
- September 24: Halo: Last One Standing, the final part of Halo: Landfall, was released.
- September 25:
- September 26: Halo 3 released in Europe.
- September 27: Halo 3 released in Japan and the rest of Asia.
- September 28: Halo 3 released in most South Korean video game shops.
- October 9: Gravesite was released digitally.
- October 30: The novel Halo: Contact Harvest was released.
- November 20: The Halo 3: Original Soundtrack was released.
- November 21: The second issue of Halo: Uprising was released.
- December 11: The Heroic Map Pack for Halo 3 was released.
- Museum first aired on TV.
- The trailer Believe: The John 117 Monument was released.
- Enemy Weapon first aired on TV.