The event, announced on the official website, will launch on October 5, 2023, at 10:00 a.m. and end on October 9, 2023 at 8:00 p.m. local time.
Some of the content is similar to the one launched in 2019 to celebrate the Detective Pikachu movie.
A bunch of pokemon will appear more frequently in the wild including Pikachu wearing a detective hat (which can evolve in the Raichu with the hat), Growlithe, Slowpoke wearing a hat, Aloan Exeggutor, Magikarp, Xatu, Bellossom, Lotad, Chimecho, Bronzor, Ducklett, Cutiefly, and Falinks.
Completing field research tasks lets you encounter Sudowoodo, Snivy, and Rowlet. You can also complete the event-themed Collection Challenge.
Taking snapshots will trigger a daily encounter with the Detective-hatted Pikachu and PokeStops hrant 2x XP during the event.
A timed research story prompts you to help Professor Willow track down a "Pokemmon of Interest" and that also will trigger an encounter with Pikachu wearing a detective hat.
Lastly, a "Detective Pikachu Avatar Pose" will be made available in the store.
While not directly related to Pokemon Go, I'll just leave this Detective Pikachu Returns ad from Japan here.
I'm pretty sure you'll have a giggle with it even if you don't understand Japanese.
Detective Pikachu Returns launches on October 6 exclusively for Nintendo Switch, while Pokemon GO is currently available for iOS and Android.