Pokémon GO - A new phase of Raid Battles begins with Cresselia!
Niantic has announced that from November 20th, 2018 until December 18th, 2018, Pokémon GO players will be able to battle Cresselia in Raid Battles.
Date + Time
Source: https://pokemongolive.com/en/post/legendary-cresselia/

A new phase of Raid Battles begins with Cresselia!
Trainers, As Pokémon’s legendary year is coming to a close, there will be an exciting appearance of a Legendary Pokémon originally discovered in the Sinnoh region. The Psychic-type Legendary Pokémon Cresselia will join Raid Battles on November 20 at 1:00 p.m. PST! Details below:Date + Time
- Nov. 20, 2018, at 1:00 p.m. PST (GMT −8) to Dec. 18, 2018, at 1:00 p.m. PST (GMT −8)
- Cresselia will appear in Raid Battles!
Source: https://pokemongolive.com/en/post/legendary-cresselia/