Rule 117.1 · Timing and Priority
Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.
Subrules
A player may cast an instant spell any time they have priority. A player may cast a noninstant spell during their main phase any time they have priority and the stack is empty.
A player may activate an activated ability any time they have priority.
A player may take some special actions any time they have priority. A player may take other special actions during their main phase any time they have priority and the stack is empty. See rule 116, “Special Actions.”
A player may activate a mana ability whenever they have priority, whenever they are casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment (even in the middle of casting or resolving a spell or activating or resolving an ability).