https://www.gamespot.com/articles/last-chance-for-pokemon-gos-kanto-week-event/1100-6458119/

A new in-game event is underway in Pokemon Go. For the next few days, Niantic is holding Kanto Week, an event celebrating the classic Red and Blue versions with increased spawn rates for some Gen I Pokemon and other bonuses. [Update: Pokemon Go's Kanto Week event is set to end tomorrow, April 17, making this your last opportunity to take advantage of the below bonuses while they're still available.]

Kanto Week runs from now until April 17. For the duration of the event, Kanto-native Pokemon, like the original starting trio Bulbasaur, Charmander, and Squirtle, will appear much more frequently in the wild. Other Gen I monsters, such as Aerodactyl and Snorlax, will also return to Raid Battles.