From Bulbapedia, the community-driven Pokémon encyclopedia.
Mirage Spots are a strange phenomena which occurs throughout the Hoenn Region. Many are random and spontaneous, while others are the result of certain Pokémon being present within the general area. These anomalies can manifest themselves in one of several ways: mysterious floating rings, rifts within the Space-Time Continuum, unusual localized weather phenomena, and the sudden appearance of entire islands and other landmasses.
Once every 24-hours, a Mirage Island will spawn into existence. At any time newer islands can emerge via StreetPass; however, they are all bound by the same timeline, thus they will only exist for the remainder of the current 24-hour cycle.