To celebrate the recent update to Mega Evolution in Pokémon GO, for one day only, Trainers will be able to use Mega-Evolved Pokémon in the GO Battle League’s Master League! The Pokémon GO Battle Day: Stardust Surprise will take place on Sunday, May 29th, 2022, from 12:00 a.m. to 11:59 p.m. local time.

Shoot for the stars during GO Battle Day: Stardust Surprise

Sunday, May 29, 2022, from 12:00 a.m. to 11:59 p.m. local time

Battle with Mega-Evolved Pokémon

To celebrate the recent update to Mega Evolution in Pokémon GO, for one day only, Trainers will be able to use Mega-Evolved Pokémon in the GO Battle League’s Master League!*

We look forward to seeing what strategies Trainers come up with using their Mega-Evolved Pokémon!

*Please note: You will need to Mega Evolve your Pokémon before bringing them into battle.

Bonuses

The maximum number of sets you can play per day will be increased from five to 20, for a total of 100 battles!

4× Stardust from win rewards.

End-of-set avatar rewards

• Trainers who complete a set at any rank during this GO Battle Day will receive the Gladion’s Pants avatar item!*
• Trainers who have already received the Gladion’s Pants avatar item from the rank 20 rewards will not receive another.

Active competitions

The following leagues will be active.

• Master League
• Ultra League
• Great League

Source: https://pokemongolive.com/en/post/go-battle-day-mega-stardust-surprise/