https://www.gamespot.com/articles/last-chance-pokemon-gos-legendary-birds-back-again/1100-6461790/

The Ultra Bonus Event is now underway in Pokemon Go, bringing an influx of Gen I Pokemon with it. For a limited time, Pokemon originally found in the Kanto region will appear much more frequently in the wild. That isn't all, however; the three Legendary birds Articuno, Zapdos, and Moltres have all also returned to the game, but for only a few days. [Update: The three Legendary birds will only appear in Raid Battles until 1 PM PT / 4 PM ET today, September 20, making this your last chance to capture them if you still haven't done so.]

From now until September 20, you'll have another chance to fight the three Legendary birds in Raid Battles. While the three are available, the duration of Raid Battles has been extended to 90 minutes, giving you some extra time to battle and catch them. Niantic also says you'll have a chance of running into Shiny versions of each Legendary.