Pokemon Go is filled to the brim with events recently which is probably the only reason why Niantic didn't chuck another event at players yet. They did the next best thing though - three Legendary Pokemon are returning in the Raid Week.
Niantic posted an update recently announcing that the three Legendary Pokemon that will pop up during the raid week will be Cresselia, Kyogre and Groudon. These three legendaries and their Shiny counterparts will join the five-star raids at specific times.
Cresselia, the elegant Psychic-type Legendary Pokemon is the one that represents the crescent moon. This one is vulnerable to Bug, Ghost and Dark-type Pokemon so in case you want to give Cresselia a spanking, make sure you have one at hand.
This Legendary will be available on Monday, 27 May 2019 from 1:00 PM PDT and will withdraw on Tuesday, 18 June 2019 at the same hour.
Kyogre is the Pokemon from Sea Basin that has a constant rivalry with Groudon. This Legendary is a Water-type that is vulnerable to Grass and Electric-type Pokemon which will present your opportunity to take this oversized fish down.
It will appear on Tuesday, 18 June 2019 when Cresselia retires and will be available until 27 June 2019 at 1:00 PM PDT.
Once Kyogre is through with the raids, Groudon will take over. This is probably the time it will finally wake up from the slumber. Anyway, it is a Ground-type Pokemon and is weak to Water, Grass and Ice-type Pokemon so these three types are your way to beating the Godzilla wannabe.
It will start appearing immediately after Kyogre goes away and will be available until 10 July 2019 at 1:00 PM PDT.
While these availability windows seem to be rather generous, there is no guarantee trainers will run into them every time. Legendary Pokemon are apparently like Gandalf, they appear precisely when they mean to.
Keep in mind that Raid Week will conclude on 28 May 2019 so Cresselia will be available for two days where it will overlap with the event.