https://www.gamespot.com/articles/pokemon-go-zapdos-returning-for-one-day-today/1100-6460541/

Pokemon Go players will soon have another chance to capture Zapdos. As a reward for meeting the Global Challenges goal during Pokemon Go Fest last weekend, the Legendary bird will return to the game this Saturday, July 21, but it won't be available for long.

For a three-hour window of time, players will be able to encounter Zapdos again as a Raid Battle at Gyms. As with other Raid Bosses, you will first need to team up with other players in-person to battle the Legendary Pokemon; if you manage to defeat it, you'll then earn an opportunity to capture it.