NOMan is correct.
PHB p.291 said:
That way your attack will be triggered by a portion of the enemy’s move, and you will interrupt it and attack first.
Even more explicitly:
PHB p.268 said:
If a creature triggers your immediate reaction while moving (by coming into range, for example), you take your action before the creature finishes moving but after it has moved at least 1 square.
And chaotix42's suggestion is essentially a less formal version of what I suggested, and what I'll rule in my games. (And, given that I'm currently a player with a newish GM, what I'll suggest
he rule should it come up.) When you ready an action, if the trigger occurs then as an immediate reaction you actually do
two things:
- Take the action you readied
- Commit to delaying your next turn until just before whoever's turn it is at the moment. You do this using the normal delay-turn mechanic, and you get to do it even if you may have been stunned/dazed in the meantime (and thus couldn't ordinarily delay your turn).
Most of the time, this will work exactly like players expect the normal readying rules to work. But it'll close all the loopholes I can see, using the same careful definition of a delayed turn and how it gets split up.
I'm happy with that.
As for readying an action for the end of your own turn, that's explicitly disallowed by two things: one, the end of your turn is not "during someone else's turn" and thus you cannot take any immediate actions there; and, two, PHB p.268 & 269, "No Action" - you can't take an action of any kind (free, immediate, or otherwise) during the start of your turn or the end of your turn.
It's more obscure with regards to other peoples' turns, since neither restriction applies there... but as a DM, my own call would be that while the ambiguity of the words "triggering action" is elsewhere a problem, now that we've established that they mean the ordinary English sense of the word "action", we can use that... and thus the only permissible trigger events are actual in-game actions, not events from the sequence of play. This is supported by p.268, where it talks about how if you ready something with respect to an opponent's movement, they have to move at least 1 square first. Their turn is broken down into substeps, yes... but not an infinitely divisible continuum of substeps, merely a discrete list of small but not infinitesimal substeps, such as one square's movement.
Oh, and I also realized that "readying a Ready Action" doesn't actually
need to be disallowed - the rules already make it useless to do so. I ready an action; my turn ends (and my one-immediate-action-per-round counter resets). I take my readied action (this is an immediate reaction and uses up my one immediate per round), stating that I want to ready another action. When the second trigger comes up, I still haven't seen my turn, so my allowed quota of immediate actions hasn't reset... so I
have to miss that trigger, not take the triggered action, and not reset my initiative for a second time. No problem.
And, yes, something to keep in mind if players start to get tricky with Ready Action in your games... it does burn your immediate action. Use with caution if there are any other immediate actions you'd like to be able to take...