https://www.polygon.com/2018/11/21/18106633/cresselia-pokemon-go-legendary-raid-battle-dates

Pokémon Go players now have the chance to capture the legendary Pokémon Cresselia, one of the Sinnoh region’s rarest monsters. The psychic-type lunar princess will show up in Raid Battles as of this week, with just under a month for players to capture it.

Cresselia is set to be a Raid boss until Dec. 18 at 4 p.m. ET, although Niantic is sometimes known to extend those times or dates for legendary Raids. But with the holiday weekend upon us, now seems to be a good time to get out there and find the Pokémon.