Niantic have announced the start time for Altered Giratina's appearances in five-star raids in Pokemon Go as well as the details surrounding the Raid Hour that has a higher chance of such raids appearing.
Altered Giratina will be present for almost a full month as the raid event will kick off on Monday, 23 September 2019 at 1:00 PM PDT / 8:00 PM GMT and last until Thursday, 17 October 2019. Giratina in its Altered Forme will be present in five-star raids only.
In order to jump-start the raid craze, players will be able to find five-star raids more often on Wednesday, 25 September 2019 during the Raid Hour . This one does not have a fixed time for the entire globe but rather adheres to local time so players around the world will be able to make the most of it. The hour will go on from 6:00 PM to 7:00 PM local time.
Those who happen to be extraordinarily lucky will be able to encounter a Shiny Altered Forme Giratina. To maximise your chances of beating this Legendary Pokemon, it would be wise to prepare for facing a Ghost-type and Dragon-type adversary while "more mysteries" remain to be unravelled although Niantic didn't specify what they are just yet.
Ironically enough, Giratina is weak both Dragon and Ghost-type Pokemon, in addition to Dark, Fairy and Ice-type. On the other hand, Ghost and Dragon-type can backfire if not using the correct environmental variables but it's also important to avoid using Psychic-type. In case you are not sure where or how to start preparing, a guide on the entire Altered Giratina encounter could be a good destination for you.
Considering Pokemon Go is just calming down from an absolute deluge of events that were present during Summer 2019, a month-long opportunity to get Altered Giratina sounds like a perfect way of winding things down.