The best time for you to hold your tongue is the time you feel you must say something or bust.

The pressure to speak is at its highest exactly when speaking is least likely to help. That urgency is not a sign the point is important; it is a sign you are activated, and things said from that state tend to be about relieving the pressure rather than communicating anything. Afterwards the relief is gone and the sentence is still in the room.

A workable rule is to write it rather than say it, and then not send it for a day. Most of the time the urge passes entirely. What remains after a night is usually shorter, truer, and worth actually saying.