The fearsome Giratina is returning to Pokémon GO Raid Battles in its Altered Forme from Thursday, March 28 at 1:00pm PDT (GMT-7) through Tuesday, April 2 at 1:00pm (GMT-7).
After that point, Giratina will fight Trainers in its Origin Forme in Raid Battles instead. Origin Forme Giratina will then be available in Raid Battles until Monday, April 29th at 1:00pm (GMT-7).