From Halopedia, the Halo wiki
Avalon was one of the six continents on the planet Arcadia, a human colony in the Procyon system governed by the Unified Earth Government. It was abandoned after the Battle of Arcadia in 2531 and destroyed by the Covenant in the Fall of Arcadia in 2549.
Avalon had four existing provincial regions, the smallest amount of territories among the Arcadian continents.
It is most likely named after the island, Avalon in Arthurian legend.