https://www.gamespot.com/articles/pokemon-go-last-day-to-catch-the-legendary-rayquaz/1100-6457345/

Players who have yet to catch Pokemon Go's latest Legendary, Rayquaza, will have to act fast if they hope to add one to their teams. The Sky-High Pokemon will only be available for a few more hours, making this your last opportunity to capture one before it leaves the game.

Rayquaza will appear in Raid Battles at Gyms until Friday, March 16. Like all other Legendary Pokemon, players will first have to team up with other trainers to battle it; only after it has been defeated will you have a chance to capture one. While powerful, Rayquaza's dual Dragon and Flying typing makes it particularly susceptible to Ice-type attacks, so players should bring Ice Pokemon such as Piloswine and Lapras to battle it.