DeNA has announced the details for the next Champion Stadium challenge which will be the Unova Challenge and it will take place from April 4th, 2021 at 11:00 p.m. until May 2nd, 2021 at 10:59 p.m. in Pokémon Masters EX.

The Next Champion Stadium Challenge

The details for the next Champion Stadium challenge are planned as follows.

Unova Challenge

In the Unova Challenge, you can go up against the Elite Four and Champion of Unova during the time period listed below.

Note: For the duration of the Unova Challenge detailed below, you will not be able to take on Champion Stadium battles for other regions.
Note: Medals for regions that are not currently active will be available during the next challenge that features those regions.

Unova Challenge Date/Time

April 4, 2021 at 11:00 p.m. - May 2, 2021 at 10:59 p.m.

Important

• You can participate in the Champion Stadium after you complete Main Story Chapter 10: A Reason to Battle (Normal) in single-player and collect all PML Badges.
• Medal acquisition and Hall of Fame information will be saved for each region regardless of event date/time.
• Details about the time period and featured region will be included in notifications going forward.
• The time period and featured content are subject to change without notice.

Source: https://pokemonmasters-game.com/en-US/announcements/Update_3030_0W_1