Can you cast a sorcery during an opponent's turn if you have flash?

Normally, sorceries can only be cast during your own main phase when the stack is empty and you have priority — this is the 'sorcery speed' restriction defined in CR 307.1.

However, flash overrides this restriction. CR 702.8a states that flash allows a player to cast that spell 'any time they could cast an instant.' Instants can be cast at any time you have priority (CR 304.1), including during an opponent's turn or in response to other spells and abilities.

The flash ability can appear directly on the sorcery card itself, or it can be granted by an external effect. For example, if Teferi, Mage of Zhalfir is in play and gives all spells in your hand flash, you could cast a sorcery from your hand during your opponent's end step. Similarly, a sorcery with the keyword 'Flash' printed on it works the same way.

Keep in mind that flash only relaxes the timing restriction. All other rules for casting the spell still apply: you must have priority, pay its mana cost, choose legal targets, etc. (CR 601.2).

Example: Your opponent attacks you and moves to their second main phase. You hold Temporal Mastery which has flash. You may cast it during their turn at instant speed — perhaps in response to an ability they activate — because flash grants you that timing permission.