You can now sign up and battle with Pokémon from the Sinnoh region in the upcoming Ultra Sinnoh Classic Online Competition with your copy of either Pokémon Ultra Sun or Pokémon Ultra Moon. You’ll need a Pokémon Trainer Club account to compete. After you log in, you’ll be able to read and accept the regulations on the Pokémon Global Link (PGL). You’ll also need your own Pokémon Ultra Sun or Pokémon Ultra Moon video game and a Battle Team of eligible Pokémon. Accept the regulations on the Pokémon Global Link by Thursday, February 22, at 23:59 UTC, and get ready to battle!
The competition goes from Friday, February 23, at 00:00 UTC, until Sunday, February 25, 2018, at 23:59 UTC. Trainers will be limited to 15 battles per day. All qualified participants in the Ultra Sinnoh Classic Online Competition will receive 50 Battle Points. Players must complete at least one battle to qualify. Good luck, Trainers!
Source: Pokémon Global Link