Just a fun idea I had for the Holy Day of a God of Peace.
Any army that initiates a battle on the Holy Day will lose, badly. Troops will be slaughtered, generals will die, and everything that might go wrong will go wrong for the attacking army. If a general plans an attack on that day, including trying to cheat by starting it the night before, something odd like having his horse fall on him, a random arrow hitting him in the eye, etc, WILL happen. This includes a king who tries to force his general to attack, against the generals wishes.
Basically, the God of Peace will have peace on his holy day, or he will kill off any army that tries to break it.