97,445 BCE
From Halopedia, the Halo wiki
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The date 100,000. BC was the approximate time when The Forerunners fired the Halos[1], or what the Covenant refer to as The Great Journey. 343 Guilty Spark put the date as the year 97,448 BC, though it should be noted that he measured it on Installation 04's year, not the 365.25 day Earth calendar.
The activation was supposed to end the Forerunner-Flood War. It killed all sentient life within three radii of the galactic center, including any Forerunners not located on The Ark or within Shield Worlds. The date, about 100,000 years ago, is around the time our Homo sapiens ancestors began migrating out of Africa. The date is also when the Halo 3: The Cradle of Life comics take place.
Mendicant Bias
The Forerunner AI known as Mendicant Bias was designed to battle the Flood but he was seduced by Gravemind and descended into rampancy. He allowed the Flood into the Maginot Sphere, which gave them the opportunity to attack the Forerunner's inner colonies. This forced the Forerunners to fire the rings and effectively commit "mass suicide." He later repented to John-117 through the terminals on the levels, promising to atone for his past actions by helping the Master Chief fire Halo and help him escape (how he does this is unknown).
