Petilil is making his Pokémon Go debut during the Pokémon Go Fourth Anniversary Challenges which will take place throughout the month of July. These challenges lead to the Pokémon Go Virtual Event Fest 2020 where anyone who purchases a ticket can attend a weekend in the comfort of their own Pokémon Go home activities to celebrate the game. The third week of the Fourth Anniversary Challenges begins on the 17th. July at 8 a.m. local time, and that's when Petilil will be available.
Petilil is a Grass-type Pokémon. You will only be able to capture it when the weather is sunny, and although the sun may shine in your area, the same must be valid in your Pokémon Go app. You can check if it is by the little icon at the top right of your screen while playing. Click on it, and it should tell you what kind of weather is going on, Pokémon Go. If the weather is nice, you have a good chance of seeing a Petilil spawn in your area after July 17th. Petilil is also available to hatch eggs from 5 miles away.
In the Pokémon games, Petilil would evolve into Lilligant if you held him with a Sunstone, but these are not available in the mobile game. You will probably need an Unova Stone to transform it into a Lilligant. You can acquire them by completing Research Tasks, spinning PokéStops, or completing a Seventh Day Field Research Task.
You can also catch a Flying Pikachu during the first week of the Fourth Anniversary Challenges and a Summer Style Pikachu during the third week alongside the debut of Petilil. Ducklett will debut in Pokémon Go starting July 3.