Season 0 was the playtest season for Pathfinder Society Organized Play. It ran from August 2008 through July 2009 and used the 3.5 rules set of the world's oldest roleplaying game. Season 0 scenarios are still legal for play in Pathfinder Society. GMs may find that they need to make a few adjustments to make them fully compatible with the Pathfinder Roleplaying Game, but those changes should be minor.