When an effect says “you may cast” something, how can I tell when timing rules apply?

Question

Some effects that say “you may cast” let you ignore timing rules. For example, Descendants' Path lets you cast creature spells during your upkeep. It doesn’t explicitly say that you can ignore timing rules or cast the spell as though it had flash, but it’s implicit (because otherwise the card wouldn’t work at all).

On the other hand, Emry, Lurker of the Loch has a “you may cast” effect with reminder text stating that “Timing rules still apply.” Since this is reminder text and not rules text, that means the rules text on its own already implies that you can’t ignore timing rules.

Those two are clear-cut cases, but in general, how to determine whether a “you may cast” effect lets you ignore timing rules? It seems like punctual effects let you ignore timing rules, while ongoing effects (“You may cast (…) this turn”) don’t, but I couldn’t find a reference to that in the comprehensive rules.

Answer 1

While these effects look similar, there is a key difference between the two:

Descendants' Path instructs you to cast the card as part of the effect's resolution. This is the case exempted by 116.1 and therefore ignores the timing restrictions given by the game rules (see 116.1a) , but is still subject to timing restrictions imposed by game objects (e.g. an opponents Teferi, Time Raveler would prevent you from casting it)

Emry, Lurker of the Loch creates a continuous effect that gives you permission to cast the card upon resolutuion. You may do so until the effect ends, but must follow the usual timing restrictions (116.1a).

116.1. 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.

116.1a A player may cast an instant spell any time he or she has priority. A player may cast a noninstant spell during his or her main phase any time he or she has priority and the stack is empty.

If a spell or ability instructs you to cast another spell as part of its resolution, it will be on the stack while the other spell is being cast, however, it will have left the stack by the time that spell resolves. While technically a legal target, you will not be able to actually copy the original spell or ability.

If a spell or ability grants you permission to cast another spell, it will have left the stack by the time you recieve priority to cast that spell and will not be a legal target.

Answer 2

There are two main kinds of effects, one-shot effects and continuous effects. The phrase "you may cast" appears in both of those effects. If it is a one-shot effect, you cast the spell immediately, and standard type-based timing restrictions do not apply. If it's a continuous effect, those timing restrictions do apply.

The rule for determining what kind of effect a "you may cast" instruction is is actually pretty simple:

• If the instruction is on a spell or in an activated or triggered ability, then it is a continuous effect if it has a duration ("this turn", "until end of turn", "as long as [that card] remains exiled", etc). Otherwise it is a one-shot effect.

• Any other "you may cast" effect is a continuous effect.

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The reason you can ignore standard timing restrictions only with the one-shot version of this effect is that they are not really timing restrictions, they are timing permissions. For example, rule 302.1 says

A player who has priority may cast a creature card from their hand during a main phase of their turn when the stack is empty. Casting a creature as a spell uses the stack. (See rule 601, “Casting Spells.”)

The rule permits you to cast creature spells at specific times, but it does not prohibit you from casting the spells at other times. A one-shot "you may cast" effect instructs you to cast the spell at another specific time: while that effect is resolving. A continuous "you may cast" effect, on the other hand, allows you to cast a spell within its duration, but it's not giving you any new permission to cast it at a specific time so you can still only cast it when you would normally be able to cast that kind of spell.

For contrast, you do still have to obey actual timing restrictions. Curtain of Light has the text

Cast this spell only during combat after blockers are declared.

If a one-shot effect instructs you to cast Curtain of Light outside of combat, you cannot do so, because Curtain of Light's ability explicitly restricts when you can cast it.