Legendary Lunch Hour
So...will the raids be just all Dialga? It would be a bit boring if it is.
Confirmed in Reddit, this is less of an "event" and literally just an experiment. They plan to gauge user activity and feedback and will make tweaks if they decide to continue doing these in the future.
And yes, for this one they will all be legendary raids. 10 minute egg timers, hour long raid windows.