Pokémon Spotlight Hour featuring Rattata, Shiny Rattata, 2x Evolution XP and increased spawns of XXS Rattata available in Pokémon GO tomorrow, September 24, from 6 p.m. to 7 p.m. local time

Pokémon GO will host a Rattata Spotlight Hour on Sept. 24 from 6-7 p.m. local time, with increased Rattata spawns, a chance to encounter Shiny Rattata and double Evolution XP. The event will also feature increased appearances of XXS Rattata, giving trainers an opportunity to collect a particularly small version of the Pokémon while earning extra experience from evolutions.

Source: Official Pokémon GO blog