https://www.gamespot.com/articles/pokemon-go-articuno-available-again-soon-through-n/1100-6459184/

Pokemon Go players have a handful Legendaries to add to their collections right now, but another one will soon be available in the game. Developer Niantic announced that it will roll out a new batch of research quests next week, which can lead to an encounter with the Legendary Ice-type Articuno.

Beginning June 1 at 1 PM PT / 4 PM ET, players will be able to find a new assortment of Field Research tasks when visiting Pokestops. Niantic hasn't detailed what these quests will entail, only that they'll revolve around Water-type Pokemon. By completing the quests and earning enough Stamps, players will achieve a Research Breakthrough, with the reward being another chance to capture Articuno, one of the first Legendaries to appear in the game.