[RPG] When do enemies save against the Command spell

The saving throw happens when Command targets a creature. Creatures who fail the save can follow the command on their next turn.

According to the Players Handbook, the effects of a spell happen when the spell is cast. If the spell prompts targeted creatures to make a saving throw, then the affected creatures should do so.

Saving Throws

Many spells specify that a target can make a saving throw to avoid some or all of a spell’s effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure.

When a creature is prompted to make a saving throw in response to some event (such as a spell), they make the roll immediately. Otherwise, the spell effects wouldn't be resolved, and the outcome of the spell would be undetermined. Therefore, creatures make the saving throw when they are targeted or affected by the spell.

A saving throw—also called a save—represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

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The result of a successful or failed saving throw is also detailed in the effect that allows the save. Usually, a successful save means that a creature suffers no harm, or reduced harm, from an effect.

The Glamour bard's Mantle of Majesty ability lets them cast the Command spell, which, due to awkward punctuation, may invite disagreement about when the Wisdom saving throw occurs:

The target must succeed on a Wisdom saving throw or follow the command on its next turn.

While the consequences of the spell (i.e., the target acting according to the command) don't happen right away, someone still needs to determine whether or not the target was affected. You issue the command as part of the spell, and so the saving throw should happen as part of the spell. If the targeted creature fails, then on their next turn, they will follow the command.