Note: "the trigger is specific enough to occur before the attack completes"

@AncientSwordRage But it's worth noting that the example OP used is not one of sufficient specificity. The entire question is based around the assumption that there can be a Ready trigger which occurs after a creature declares an intention to attack/cast a spell, but sufficiently before the spell/attack happens that you can still foil it. It's worth pointing out that this may not be possible.

@Gandalfmeansme Counterspell implies it's possible, even if that is the sole exception.

@AncientSwordRage There are certainly Reactions that happen upon the start of a character doing something and complete before the character accomplishes that thing (AoO, Counterspell, Shield). But according to DMG p. 252 "opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action." And since this question is about Readying movement, it's relevant to point out that the rules may not support this timing.

(By the way FenrirG, although it's certainly not essential, since your quoted rules already establish the timing of a Readied movement/action compared to its trigger, the rules I quoted from the DMG might be worth including in this answer, just to clarify that Ready triggers in a different way than some other Reactions [i.e. it can't interrupt its trigger, while other specific ones can]).

Ah, but the distinction I think matters is, if the trigger is "The mage starts casting" vs "The mage has finished casting", not whether you can complete a reaction before the latter trigger 'resolves'.

@AncientSwordRage It's hard for me to picture a time that is after "the mage starts casting" or "the enemy starts an attack" which is something other than "the mage casts the spell" or "the enemy attacks." Unless the attack or spell takes an unusually long time, of course (i.e., unless it takes more than an action).

There are certainly triggers you could use that could remove an enemy's ability to attack you: for example, maybe you notice that the enemies are armed with hand crossbows. You could say "I retreat 30 feet if an enemy gets to within 120 feet of me." At that point, an enemy could get within 120 (trigger), and after that, you retreat 30 feet. You are no longer in range of the enemy's weapon, so they cannot attack (without moving further)

But the question is assuming that you are not only moving in a way that makes attacking/being targeted by a spell impractical, but that you do so after the attack/spell begins. Like I said, I think that in most conceivable situations "after the attack begins" is "the attack."

(It would be like saying "after he begins speaking, but before he speaks": it feels like there is no such timing)

And if we argue that the two things (movement and the attack) are happening simultaneously (which is a possible interpretation), then Xanathar's gives us guidance that says that the player/DM controlling the creature whose turn it is will decide which event (movement or attack) happens first. And since this is readied movement, it's almost definitely the attacker's/spellcaster's turn.

@Gandalfmeansme when they've opened their mouth but before any sound comes out?

c.f. an unofficial ruling by JC: sageadvice.eu/can-silence-interrupt-a-spell-caster

> As DM, I'd allow a readied silence spell to interrupt the casting of a spell with a verbal component.

Maybe that's dumb, but it's possible

@AncientSwordRage Great evidence! The "as a DM" statement moreso indicates that this is something which isn't necessarily supported by the rules, but it's a very strong piece of evidence suggesting this is possible!

What about after he begins speaking but before he finishes his sentance?

Unfortunately, the rules tell us that you can react to perceivable triggers, but provide few examples for determining how finely a trigger can be parsed

At last in some cases, you can react to a spell casting before the spell is complete (counterspell, shield)

What is not clear is whether this is a rule or an exception