The rules for the Ready action are as follows:
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
Some context:
My group's bard got the brilliant idea to cast minor illusion on an Umber Hulk to surround its head in a 5x5 black cube, thereby making it impossible for party members to see it (and consequently suffer the effect of Confusing Gaze) until it moves again.
We were recently debating the idea of combining this with the Ready action. The intent is to declare, "When the umber hulk ends its turn, I cast the illusion on its head, effectively neutering its Confusing Gaze round after round."
Would that be possible, or is a more in-fiction trigger required?
