Niantic has shared a few details about the event, which kicks off on Tuesday, November 16 at 10am local time with Part 1: Pokemon Brilliant Diamond. The first event will have Turtwig, Chimchar, and Piplup wearing Lucas' hat in the wild, and you'll be able to find Pokemon wearing Dawn's hat by completing Field Research tasks before Part 1 ends on Thursday, November 18 at 8pm. During Part 1 of the event, Seel, Murkrow, Poochyena, Aron, Bidoof, Kricketot, Bunear, Stunky will appear more often in the wild, and you might even get lucky enough to encounter a Shieldon.
Part 2 of the event, themed around Pokemon Shining Pearl, will feature the same Pokemon wearing special hats, but the hats are flipped, so Turtwig, Chimchar, and Piplup will be wearing Dawn's hat, and you'll find Pokemon with Lucas' hat in Field Research tasks. The Shining Pearl event will also have the following Pokemon appearing more often in the wild: Pinsir, Misdreavus, Houndour, Spheal, Bidoof, Kricketot, Buneary, Glameow. Some less likely, but still possible encounters during the second part of the event include Slowpoke, BagoBuizel, and Burmy (Sandy Cloak). Pokemon Go's Shining Pearl event runs from Thursday, November 18, at 10am local time to Saturday, November 21 at 8pm local time.