Apologize while the person is still close enough to hear it.

An apology loses nothing by arriving early and can become impossible by arriving late. People move, stop answering, or are simply gone, and after that the sentence has nowhere to land and stays with you instead. Waiting for the right moment is usually waiting to feel less embarrassed, which does not happen on its own.

Pick the one you have been carrying and keep it short. Name the specific thing, say you are sorry, and leave off the explanation that turns it into a defense. Whether they accept it is theirs. Saying it was yours.