Season 1

Season 1 of GO Battle League lasted from March, 2020 to May , 2020. It is preceded by Preseason and succeeded by Season 2.

Formats

 * Battle League Great.png Great League: March, 2020 to March , 2020
 * Battle League Ultra.png Ultra League: March, 2020 to April , 2020
 * Battle League Master.png Master League: April, 2020 to April , 2020
 * GO Battle League.png All three Leagues: April, 2020 to May , 2020

Ranks and Ratings
The requirements were different from Preseason.

Rewards
Basic Premium

Stardust

The random items included:
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Reaching rank 10 could reward avatar items and an avatar pose inspired by Steven.

Pokémon
Metagross was the first guaranteed encounter. Pikachu Libre was the first encounter in rank 10, one only.

Certain Legendary Pokémon or Mythical Pokémon could be obtained starting at rank 4 during their time in tier 5 Raid Battle.

End-of-season rewards

 * (rank 7 or above)

Events

 * GO Battle League Season 1 Launch Celebration was held to promote Season 1.
 * When Season 1 was initiated, the rank and rating of Trainers were slightly altered from that in Preseason. Later, Niantic adjusted all Trainers to rank 1. If a Trainer claimed the end-of-preseason rewards during this time slot, their Preseason record would be lost.
 * Due to the COVID-19 pandemic, Trainers do not require walking or PokéCoins to unlock battle sets since the start of Season 1.
 * It was originally planned to end on April and then May . The bonus is still active for today.
 * Since April, if one lost all 5 matches in a set, they could continue playing in that same set until scoring a win for up to 15 losses, after which they would move to the next set.
 * This feature was removed in Season 3.
 * GO Battle Day: Marill took place on April, 11 a.m. to 2 p.m. local time. Marill appeared as a guaranteed Basic Reward after the first and third wins, or after every win if using a Premium Battle Pass. Trainers also received double Stardust for catching Marill during this time period.
 * For the whole day, Trainers could complete 20 sets of battles.
 * Season 1 was originally planned to end on May, but due to certain technical issues, it was delayed until May.

Maintenance

 * March, 2020 - soon after the start of Season 1, GO Battle League was closed for 20 minutes to adjust rank and rating
 * On the same day, it was down for 3 to 5 hours again.
 * March, 2020 - temporarily closed for 1 to 3 hours
 * March, 2020 - temporarily closed for 1 to 3 hours when Great League ended
 * March, 2020 - temporarily closed for 1 to 3 hours
 * April, 2020 - temporarily closed for 1 to 3 hours
 * April, 2020 - temporarily closed for 1 to 3 hours
 * April, 2020 - temporarily closed for 1 to 3 hours when Ultra League ended
 * April, 2020 - temporarily closed for 3 to 5 hours
 * April, 2020 - temporarily closed for 3 to 5 hours
 * April, 2020 - temporarily closed for 2 to 4 hours
 * April, 2020 - temporarily closed for 1 to 2 hours
 * May, 2020 - temporarily closed for 2 hours
 * May, 2020 - temporarily closed for 5 hours
 * May, 2020 - temporarily closed for 3 hours