Can you cast an instant during the untap step?

The untap step is the first step of each turn's beginning phase, but it is special in one critical way: no player ever receives priority during it. Because casting a spell or activating an ability requires a player to have priority (CR 116.1), nothing can be cast or activated during the untap step — not even instants or abilities with flash.

This is explicitly stated in CR 502.3: 'No player receives priority during the untap step, so no spells can be cast and no abilities can be activated.' The step simply executes its actions (phasing and untapping permanents) and ends automatically.

Priority is first granted at the start of the upkeep step (the very next step), which is the earliest point in a turn where players may cast instants or activate abilities. CR 503.1 confirms that the active player receives priority at the beginning of the upkeep step.

Concrete example: Your opponent controls a Stasis that keeps their permanents tapped. You cannot cast an instant like Shock during your untap step hoping to exploit some timing window — you must wait until at least the upkeep step before you receive priority and may cast anything.

There are also no triggered abilities that can be placed on the stack during the untap step, since triggers are only checked for placement when a player would receive priority (CR 117.5). Any trigger that fires during untap waits until players next receive priority in the upkeep step.