Preseason

Preseason of GO Battle League lasted from January, 2020 to March , 2020. It is succeeded by Season 1.

According to Niantic, Preseason is a period of time before a season of competitive play begins. They will be able to optimize and balance the GO Battle League feature during this time with trainers help. They will also be taking this time to learn about how the GO Battle League is being used in order to determine things like ideal length of seasons, rating thresholds for ranks, and more.

Formats

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

Master League was originally planned to end on March and then the format would return to Great League, but this was changed to all three Leagues.

Rewards
Before March, 2020, the Rare Candy given from each set of reward was 1 and 7 respectively.

Basic Premium

Stardust

The random items included:
 * or.
 * or.
 * or.
 * or.

Reaching rank 7 could reward avatar items inspired by Pikachu Libre.

Pokémon
Snorlax was the first guaranteed encounter and Scraggy was the first encounter in rank 4 if Snorlax had already be encountered.

Since March, 2020, certain Legendary Pokémon or Mythical Pokémon could be also obtained starting at rank 4 during their time in tier 5 Raid Battle.

End-of-season rewards

 * (rank 7 or above)

Events

 * Initially, unlocking a set required to walk 5 km and at least 2 km was needed first to unlock via PokéCoins.
 * Due to the COVID-19 pandemic, GO Battle League had the following amendments in Japan, South Korea and Italy from March to, 2020:
 * without walking or PokéCoins to unlock battle set
 * receive more Rare Candies
 * higher chance to encounter Scraggy and Legendary Pokémon

Maintenance

 * January, 2020 - temporally closed for 1 to 3 hours
 * February, 2020 - temporally closed for at least 16 hours
 * February, 2020 - temporally closed for 3 hours
 * February, 2020 - temporally closed for 30 minutes
 * February, 2020 - temporally closed for 20 minutes
 * February, 2020 - temporally closed for 30 minutes
 * February, 2020 - temporally closed for 30 minutes
 * February, 2020 - temporally closed for 30 minutes
 * February, 2020 - temporally closed for 30 minutes
 * March, 2020 - temporally closed for 45 minutes
 * Later, it was down for 1 to 3 hours again.
 * March, 2020 - temporally closed for 1 to 3 hours
 * March, 2020 - temporally closed for 1 to 3 hours