Niantic has announced that the Pokémon GO Battle League Season 1 starts on Friday, March 13th, 2020, at 1:00 p.m. PDT (GMT −7). Also, for a limited time, Darkrai, Altered Forme Giratina, and Thundurus will be coming to the GO Battle League as rare reward encounters.

GO Battle League Season 1

Trainers, The GO Battle League preseason is coming to an end, and we’re excited to announce that GO Battle League Season 1 officially starts this month on Friday, March 13, 2020, at 1:00 p.m. PDT (GMT −7)! From Trainers who gave the GO Battle League a try to earn Sinnoh Stones for Rhyhorn Community Day to battlers who climbed all the way to the top at rank 10, we thank you. We’ve been working on improving Pokémon GO’s global online battling system, and it’s all thanks to your participation and feedback.

As a token of our gratitude, Trainers who achieved rank 4 or higher will receive a Premium Battle Pass as part of their end-of-preseason rewards. To celebrate the end of the preseason, from today, Thursday, March 5, 2020, at 1:00 p.m. PST (GMT −8) to Friday, March 13, 2020, at 1:00 p.m. PDT (GMT −7), all three leagues will be available to play in the GO Battle League! Pick and choose between the Great League, Ultra League, and Master League, and end the preseason however you’d like. As we continue to work on the GO Battle League experience, let us know what you think!

For those of you who haven’t had a chance to try it—what is the GO Battle League? Like many Pokémon GO features, the GO Battle League encourages players to get out and explore the world with Pokémon. You can walk to earn entry into the GO Battle League, then battle opponents from around the globe through an online matching system and rise in the ranks of the league. This feature was a way to make the Pokémon GO battling experience more competitive and accessible to more Trainers.

Note: In-game events for may vary in the following areas until further notice. Players in these areas can stay tuned to our channels for updates.

• Japan
• South Korea
• Italy

Legendary and Mythical Pokémon are coming to the GO Battle League!
For a limited time, Darkrai, Altered Forme Giratina, and Thundurus will be coming to the GO Battle League as rare reward encounters, starting at rank 4! Darkrai and Altered Forme Giratina especially can be powerful additions to your Ultra League and Master League teams, so be sure to catch them if you can!

• You’ll have a chance to encounter Darkrai as a GO Battle League reward encounter from Friday, March 6, 2020, at 8:00 a.m. to Monday, March 9, 2020, at 10:00 p.m. local time. If you’re lucky, you might encounter a Shiny Darkrai!
• You’ll have a chance to encounter Altered Forme Giratina as a GO Battle League reward encounter from Friday, March 13, 2020, at 8:00 a.m. to Monday, March 16, 2020, at 10:00 p.m. local time. If you’re lucky, you might encounter a Shiny Altered Forme Giratina!
• For all other time periods, you’ll have a chance to encounter the Legendary Pokémon Thundurus as a reward encounter in the GO Battle League until further notice.

Warm up for GO Battle League Season 1 with two special events
The greatest Trainers know to prepare before a tough battle! Leading up to and at the beginning of Season 1, look forward to the following GO Battle League–themed events to help you catch strong Pokémon to use in battle and gain more Stardust to power up your Pokémon. Please note, the event-exclusive Pokémon appearing in raids and in the wild will not occur in Japan, South Korea, and Italy. Event bonuses will still occur in Japan, South Korea, and Italy.

• An event to help you prepare for GO Battle League: From Tuesday, March 10, 2020, at 8:00 a.m. to Thursday, March 12, 2020, at 10:00 p.m. local time, strong Pokémon will be appearing in raids, including Skarmory, Swampert, Shieldon, and more! Registeel and Cresselia will be appearing in five-star raids. You’ll also earn twice the usual Stardust from Raid Battles and Trainer Battles during this event. Please note that this bonus will not apply to GO Battle League Trainer Battles.
• A GO Battle League Season 1 launch celebration: From Friday, March 13, 2020, at 8:00 a.m. to Monday, March 16, 2020, at 10:00 p.m. local time, more Pokémon thought to be powerful in the GO Battle League will be available in raids, including Altered Forme Giratina in five-star raids. Fighting-type Pokémon will also be appearing more frequently in the wild! Also if you’re lucky, you might find a Shiny Timburr! Your best bet would be battling and catching it in one-star raids! There will also be the following GO Battle League bonuses during the event period.
  • You’ll receive a Premium Battle Pass when you win all five battles in a set on the premium track of the GO Battle League.
  • You’ll be able to complete seven sets of GO Battle League battles per day, up from the regular five sets, for a total of up to 35 battles a day!
  • You’ll receive bonus Stardust in the GO Battle League.

GO Battle League Season 1 starts on Friday, March 13, 2020, at 1:00 p.m. PDT (GMT −7)
We’re excited to announce the following changes to the GO Battle League for Season 1.

• We’re fine-tuning the GO Battle League’s rewards. Depending on your rank, you’ll receive increased Stardust rewards and a different number of Charged and Fast TMs.
• Winning matters much earlier in the ranks now. Ranks 2 and 3 now require you to win a certain number of battles to advance to the next rank. Previously, wins mattered only beginning at rank 4.
• The Pokémon you can encounter as GO Battle League rewards will change for Season 1. Metagross will be your guaranteed first encounter reward for Season 1. At rank 4, you’ll have a chance to encounter Beldum. At rank 7, you’ll have a chance to encounter Rufflet, making its Pokémon GO debut ahead of appearing in Safari Zone Philadelphia. And last but not least, the tough Pikachu Libre will be making its debut as a guaranteed reward encounter when you reach rank 10! In the future, Pikachu Libre will be available starting at lower ranks, but this season is your chance to challenge yourself to see if you can make it to rank 10, Trainers. Machop, Snorlax, and Meditite will no longer appear as GO Battle League encounters, but you’ll encounter Scraggy more often. These reward encounters might change next season, so be sure to give it your all and climb as high as you can in the rankings, Trainers!
• Reach rank 10 to earn avatar items and an avatar pose inspired by Steven. Steven is a Pokémon League Champion in the Hoenn region. Steven is both an incredibly skilled Pokémon Trainer and a kind and helpful mentor. His dedication to battle makes his avatar items and avatar pose rewards a perfect match for the GO Battle League. The pose is a reward exclusive to Season 1.
• At the end of Season 1, you’ll receive an Elite Charged TM if you’ve reached Rank 7 or higher. Introducing Elite Charged TMs and Elite Fast TMs! By using an Elite Charged TM or Elite Fast TM, you’ll be able to select the attack that your Pokémon learns. Not only that, but Elite Charged TMs and Elite Fast TMs will also allow Pokémon to learn attacks previously available only during events such as Raid Days or Community Days.

GO Battle League Season 1 timeline
Season 1 will rotate through leagues, just like the preseason did. Season 2 is set to begin in early May, but please note that all these dates are still tentative. Stay tuned—we’ll update you as soon as anything changes.

• The Great League will run from Friday, March 13, 2020, at 1:00 p.m. to Friday, March 27, 2020, at 1:00 p.m. PDT (GMT −7).
• The Ultra League will run from Friday, March 27, 2020, at 1:00 p.m. to Friday, April 10, 2020, at 1:00 p.m. PDT (GMT −7).
• The Master League will run from Friday, April 10, 2020, at 1:00 p.m. to Friday, April 24, 2020, at 1:00 p.m. PDT (GMT −7).
• All three leagues will be available from Friday, April 24, 2020, at 1:00 p.m. to Friday, May 1, 2020, at 1:00 p.m. PDT (GMT −7).
• Season 2 will begin Friday, May 1, 2020, at 1:00 p.m. PDT (GMT −7).

Source: https://pokemongolive.com/en/post/gobattleleague-season1/