The official Pokémon website has announced that Darkrai, Altered Forme Giratina, Cobalion, and Lugia return to Pokémon GO in March 2020 for four Legendary Special Raid Weekends with an opportunity to encounter Shiny Pokémon.

Darkrai, Altered Forme Giratina, Cobalion, and Lugia Return to Pokémon GO

This March, enjoy four Legendary Special Raid Weekends—some featuring event-exclusive moves—with an opportunity to encounter Shiny Pokémon.

March is shaping up to be an exciting month in Pokémon GO. A Legendary Special Raid Weekend will run every weekend, with a different Legendary Pokémon appearing in five-star raids each time. During these events, you'll receive an attack boost when you raid with friends. Plus, if you're lucky, the Legendary Pokémon you encounter might be Shiny.

The Legendary Special Raid Weekends take place each weekend in March from Friday at 8:00 a.m. until Monday at 10:00 p.m. in your local time zone. Here's the schedule for the entire month:

Darkrai: Friday, March 6th, 2020 – Monday, March 9th, 2020
Altered Forme Giratina: Friday, March 13th, 2020 – Monday, March 16th, 2020
Cobalion: Friday, March 20th, 2020 – Monday, March 23rd, 2020
Lugia: Friday, March 27th, 2020 — Monday, March 30th, 2020

There are also a couple of extra twists to this month of Legendary Special Raid Weekends. Cobalion encountered will know the event-exclusive attack Sacred Sword, and Lugia will know the event-exclusive attack Aeroblast!

With so many powerful Pokémon to face in March, you'd better get a good team lined up. And remember to venture out with your friends to get that sweet, sweet attack boost. Have fun, Trainers, and good luck!

Source: https://www.pokemon.com/us/pokemon-news/darkrai-altered-forme-giratina-cobalion-and-lugia-return-to-pokemon-go/