From Halopedia, the Halo wiki
Adelaide is a human colony world.
Within 72 hours after October 17, 2558, a Guardian Custode awoke under the planet's surface. The awakening caused massive damage to a dense city on the planet and enemy resistance was detected in the area afterward. The event happened within the same window of time to similar events on Conrad's Point, Ursa IV, Queen Anne VI, and Oban.