Friday is the best day to drop bad news - since it delays the major news networks from getting live pundit reactions on it. If he stepped down on Friday they could potentially get a replacement announcement (and media circuit on late night spots) running starting on Monday next week.

Holding off until Friday makes a lot of sense though my gut says an announcement immediately after the debate would likely have been more effective. That may have possibly created optics that Trump “won” the debate though.