The rules on Readying a spell are as follows (PHB, p. 193, bold added)
When you ready a spell, you cast it as normal but
hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends…
Up until now, I'd never really asked myself this, but can you Ready a spell which also requires concentration? Or put another way, does a spell's requirement of concentration activate when it is cast, or when it is "released"?
As an example, you ready the spell "Web" to cast it as soon as three or more enemies are within the same 20 foot cube. Does the fact that the spell requires concentration mean it automatically fails (you are both concentrating on the spell, and concentrating on readying it, which you can't do), or does the concentration on the spell only start after you spend your reaction to "release" the spell?
At first, I though this was a silly question, but Jeremy Crawford has stated elsewhere that:
A concentration spell ends the instant you start casting another concentration spell,
This implies that concentration begins when you "cast" the spell, not just when you release it. So now I'm not sure. Can you ready a concentration spell, or do the two "concentration" requirements make this impossible?
Best Answer
You can ready a concentration spell
When you ready a spell you completely cast it and hold the energy but none of the effects of the spell happen until it is released.
The spell-specific concentration aspect doesn't take effect until its release just like the duration and other effects/attributes don't start taking effect or counting before it is released. The spell-specific concentration starts when the duration of a spell starts. And the duration of a spell starts when the spell takes effect (so upon release in this case).1
Thus, since there is only one source of concentration active when holding a spell, there is no conflict and the Ready action proceeds as normal.
Also, regardless of the above logic, if Ready was meant to exclude concentration spells it would have come out and said so instead of relying of really minute logical inconsistencies to do so.
The Crawford ruling doesn't change anything here
Regarding the Crawford Tweet looking at the full sentence reveals its context and why it does not affect this case:
The ruling doesn't matter here because there is only one spell and one source of concentration happening at a time when you ready a concentration spell.
1 - If duration was counting during the held part of readying then it would be impossible to Ready instantaneous spells because their duration would have expired the instant after you started holding their energy. And the book explicitly gives the example of readying magic missile as a valid ready action. Thanks @Gandalfmeansme for this suggestion.