344. A Showdown begins when Control of a Battlefield is Contested during a Cleanup and the turn is in a Neutral Open State.
344.1. If Control of a Battlefield is Contested between two players, then a Showdown will be opened as the first step of Combat. If a Showdown is already ongoing at that Battlefield, it will become a Combat Showdown and a Combat will initiate there. See rule 459. Combat for more information.
344.2. If Control of a Battlefield is Contested, there aren't units controlled by different players there, and the turn is in a Neutral Open State, a Showdown is opened during the next Cleanup. See rule 318. Cleanups for more information.