It is not an event, it is the post-game.
The Delta Episode (Japanese: エピソード デルタ Episode Delta) is a mission following the player's first completion of the Hoenn League in Pokémon Omega Ruby and Alpha Sapphire. The episode revolves around a meteoroid that is on course for collision with the planet and a newly developed character, Zinnia.
Basically, Gamefreak added special post-game content to make catching Rayquaza and Deoxys more enjoyable. It can only be accessed after you beat Steven Stone for the first time.
Source: Bulbapedia and Experience.