Just thinking about the famous WW2 (WW1?) Christmas Truce, was it considered dishonorable to attack on Christmas Day? Did the British try to use this as anti-Revolutionary propaganda?
To clear up your confusion, the Christmas truce was during the First World War, in 1914, mostly in Flanders.
The Christmas Truce was not formal. By that I mean that no nations signed a treaty saying they would participate in soccer games in no man's land that day.
It was early enough in the war that hostilities hadn't really opened yet, so there wasn't a lot of animosity between the Allied and Central powers yet. The war started kind of suddenly, with the assassination of someone that the French and Germans and English didn't really care about. The foot soldiers didn't have any reason to hate each other yet. So on Christmas they jokingly challenged each other to play soccer and ended up playing.