https://pokemongolive.com/en/post/globaldowntime-jun2020/
May 22, 2020
Pokémon GO will be down on Monday, June 1, 2020, from 11:00 a.m. to 6:00 p.m. PDT (GMT −7)
Pokémon GO
Trainers,
Pokémon GO will be experiencing global downtime for seven hours on Monday, June 1, 2020. From around 11:00 a.m. to 6:00 p.m. PDT (GMT −7), the game will be undergoing server maintenance and will be inaccessible to all Trainers. We apologize in advance for any inconvenience that this might cause.
Be sure to follow us on social media, opt in to receiving push notifications, and subscribe to our emails to stay updated. For the latest on in-game events and feature updates, be sure to check this help center article.
—The Pokémon GO team
Should be from 2 June 2020 2am to 9am Singapore time, if I'm not wrong.