2020-04-17 : If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.

2020-04-17 : The converted mana cost of a token that isn’t a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.

2020-04-17 : You choose odd or even while Extinction Event is resolving. Players can’t take any actions between the time you make that choice and the time some creatures go extinct.