Niantic has announced that from Monday, September 23rd, 2019 to Thursday, October 17th, 2019, Giratina in its Altered Forme will be available in five-star raids.

Altered Forme Giratina—raids, Raid Hour, and Shiny release!

Trainers, We are excited to announce that Giratina has reemerged from the Distortion World and returned to raids! Date + Time

• From Monday, September 23, 2019, at 1 p.m. PDT (GMT −7) to Thursday, October 17, 2019, at 1 p.m. PDT (GMT −7): Giratina in its Altered Forme will be available in five-star raids.
• On September 25, 2019, from 6:00 p.m. to 7:00 p.m. in your local time zone: There will be an increased number of five-star raids, featuring Altered Forme Giratina.

Features

• If you’re lucky, you might encounter a Shiny Altered Forme Giratina!

Start prepping your teams to take on this Ghost- and Dragon-type Legendary Pokémon. There are still many mysteries to be unraveled about the Renegade Pokémon! Stay safe, and happy raiding!

Source: https://pokemongolive.com/en/post/shinyalteredformegiratina2019/