Flash is a static ability that allows a permanent spell to be cast at any time you could cast an instant (CR 702.8a). Normally, creature spells can only be cast during your main phase when the stack is empty and you have priority (CR 307.1). Flash overrides this restriction entirely.
During your opponent's end step, the active player (your opponent) receives priority first, but once it passes to you, you may cast a Flash creature spell. The end step is a valid time to cast instants and spells with Flash (CR 509.1, 509.2).
This is a popular strategic choice: casting a Flash creature at end of turn means it's available to attack on your very next turn, and your opponent has fewer options to respond during their own end step compared to your upkeep or draw step.
Example: Your opponent says "Go" and passes priority in their end step. You cast Rattleclaw Mystic — wait, that has no Flash — but imagine it had Flash. You pay its cost, put it on the stack, it resolves, and it enters the battlefield. Now on your turn it's already untapped and ready to attack or use abilities.
Note: If any effect says "you may cast [creature] only during your turn" or similar, that would override Flash (CR 702.8b). Also, if your opponent's end step triggers abilities that clog the stack, simply wait for the stack to clear before casting.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.