Niantic has announced that Giratina will return to raids in Pokémon GO from March 28th, 2019 until April 2nd, 2019.

Giratina Returns, Ready to Transform!

Trainers, You did your team leaders proud back when you battled Giratina in its Altered Forme. However, there are still many mysteries to be unlocked about the Renegade Pokémon. Giratina has reemerged from the Distortion World to renew and transform. Starting March 28, Altered Forme Giratina will return to raids until April 2, when it will transform into Origin Forme to keep testing Trainers’ strength until April 29.

Date + Time

• Giratina (Altered Forme): Thursday, Mar. 28, 2019, at 1 p.m. PDT (GMT −7) to Tuesday, Apr. 2, 2019, at 1 p.m. PDT (GMT −7)
• Giratina (Origin Forme): Tuesday, Apr. 2, 2019, at 1 p.m. PDT (GMT −7) to Monday, Apr. 29, 2019, at 1 p.m. PDT (GMT −7)

Start prepping your teams to take on this Ghost- and Dragon-type Legendary Pokémon originally discovered in the Sinnoh region. Stay safe, and happy exploring!

Source: https://pokemongolive.com/en/post/giratinareturns2019/