Typically, when using the Ready action to cast a spell, the following rule applies (emphasis mine):
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.
Now, suppose I have a Wand of Polymorph. On my turn, I take the following course of action:
I take the Ready action to use the Wand of Polymorph on the Roc if it flies within 15 feet of me.
If I were just using the wand on my turn to cast polymorph, I would have to concentrate as usual, since the rules for Activating an Item state:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell and caster level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
Since the action I am readying is not the Cast a Spell action, it is the "activate the Wand of Polymorph" action, do I have to concentrate to "hold its energy" until I release the spell on the Roc?
Best Answer
No, you do not have to maintain concentration when readying the use of a magic item to cast a spell.
The Ready action states (emphasis mine):
This rule applies when you specifically ready a spell, not cast a spell. You might be using the magic item to cast a spell but you are readying the use of the magic item, not readying a spell. As such the above rule does not apply.
This is a rather loose explanation and has already lead to a question in the comments, so I will include a more formal one:
To ready a spell means you are taking the Ready action, then choosing to ready the Cast a Spell action. Per the rules of the Ready action, the Casting Time of the spell has to be one action. This Casting Time is just that, a casting time, it is not its own action separate from the Cast a Spell action. This is why every magic item which allows the casting of a spell always includes something like "you can use an action to..." which is a special action you can take granted by the magic item, distinct from the Cast a Spell action. You are readying the use of this special action from the magic item, because spells themselves do not have their own individual actions. And since this special action is not the Cast a Spell action, the rules from the Ready action pertaining to readying spells do not apply here.
Although Jeremy Crawford's tweet are not considered official rulings, he has answered this question here.