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?