The Battle of Kholo was a Covenant assault on the human colony world of Kholo sometime in 2539. The Covenant learned the location of Kholo from the data banks of a recovered human freighter, and sent the Fleet of Righteous Vigilance to destroy the colony. Kholo was only protected by a handful of warships in orbit, and the Covenant scout ships were able to destroy them before the fleet even arrived. The planet was quickly glassed, and very few civilians were able to be evacuated during the conflict.
These events are referenced in the short story The Return from the anthology Halo: Evolutions.