Must be a public, non-sarcastic apology for something he hasn't apologized for in the past. 

In cases of ambiguity, this question uses definition 1a from 

an admission of error or discourtesy accompanied by an expression of regret

Pleading guilty to a crime does not necessarily count as an apology, but there may be situations in which Trump does apologize for something in court and those would count even if he's just doing it for legal reasons.

These rules may be updated to better cover edge cases, so please bring up any you think of in the comments. I will not trade in this market, except to set the initial probability.

This market resolves to earliest time period in which Trump apologizes for something, or to "Not before Election Day" if he doesn't by then.