Can a player hold the effect or spell granted to them by an item, and if so, does the spell or effect follow the normal "Readying spell" rule where if the trigger never occurs, the effect or the spell, or the charges on the item are wasted.
IE: "I hold the effect of my Cape of the Mountebank (dimension door) until the minotaur moves into melee with me."
The minotaur never moves into melee with said character.
Best Answer
Cape of the Mountebank (DMG 157)
Activating an Item (DMG 141)
Ready (PHB 193)
Using the above quotes (with some unrelated text omitted, and with my emphasis in bold), I see this situation with the following logic:
Therefore, if the trigger does not occur, the spell is lost. You must wait until the next dawn to use it again.
Addendum
Note that this question explicitly asks what happens when the effect of a magic item is held via the Ready action. For the more general case of readying the use of a magic item, see this question.