Registration for the Kanto x Alola Regional Rumble is officially open for Pokémon Sun and Moon players. To register for the upcoming Online Competition, log on to your account from the Pokémon Global Link (PGL) homepage and select Online Competitions in the main menu. You’ll then be able to read and accept the regulations on the PGL. You’ll need a Pokémon Trainer Club account, your own Pokémon Sun or Pokémon Moon video game, and a Battle Team of eligible Pokémon. Accept the regulations on the Pokémon Global Link before Friday, March 17, at 00:00 UTC, and you’ll be good to go!
The competition is scheduled to go from Friday, March 17, at 00:00 UTC, until Sunday, March 19, at 23:59 UTC. The leaderboards will be posted after the competition ends. Trainers will use Pokémon from the Kanto and Alola regional Pokédexes to create teams for Single Battle matches. Battles will take place using your Pokémon Sun or Pokémon Moon game. All Pokémon will be set to Lv. 50 for the duration of the match. See the regulations for a complete list of eligible Pokémon. Qualified participants will receive a Pidgeotite and a Steelixite for playing.
Source: Pokémon Global Link