You'll also be able to catch a shiny Ralts that's blue where the regular one is green. Pokemon Go Hub reported that shinies will appear in roughly one out of every 24.5 encounters. And if you evolve any Ralts into final forms Gardevoir or Gallade (the latter requires a Sinnoh Stone) up to one hour after the event, it'll learn the exclusive Psychic-type move Synchronoise.
During the event, any lures set to attract wild Pokemon will last three hours and eggs you start incubating will hatch in a quarter of the time. Note that it won't apply to eggs you have in incubators prior to the event, so don't set things up beforehand.
You can also catch shiny Rayquaza in five-star raids until Sept. 2 at 1 p.m. PT.
First published July 25 at 4:20 a.m. PT.
Update, Aug. 1 at 3:10 a.m. PT: Adds details on exclusive move Synchronoise.