MTG Arena will run the new ruling for one week during the War of the Spark Chronicles Singles tournament. The goal is to give players a chance to test out the London Mulligan rule while catching glitches and bugs.
Wizards of the Coast is implementing a new mulligan system, called the London Mulligan rule, with the release of the Core Set 2020 expansion in July. To ensure the ruling goes off without a hitch on MTG Arena, all players are encouraged to join in the testing of it during the WotS Chronicles Singleton tournament (June 7 to 14).
The London Mulligan rule is an improvement upon the current system, allowing players to draw seven cards with each mulligan. Depending on the number of mulligans a player takes, they then discard that many cards from a hand of seven before beginning a match.
Testing during the War of the Spark Chronicles Singleton event gives players a chance to get a feel for the new system and witness its various advantages.
For example, a well mana-curved opening hand in Singleton could determine a match. Previously, players would mulligan and hope for the best. Now they can draw seven cards and choose which ones to discard, preventing mana and curve issues.
The launch date of Core Set 2020 and the London Mulligan on MTG Arena and Magic Online is July 2. Testing of the new system will occur between June 7 to 14 during the War of the Spark Singleton tournament. The official date for the London Mulligan rule to go into effect in paper form is during the Core Set 2020 pre-release weekend, July 5 to 6.