The christmas truce of world war i has been called one of the most unusual human events in all of history. why do you think this is so?

The truce happened in the middle of heavy fighting. This means that the men who were fighting each other were ready to kill each other the previous and the following days. Yet, during the time of the truce they didn't.

This was also in clear violation of the orders, which soldiers rarely disobeyed.

Finally, all the soldiers agreed to to it at the same time: this was a joint action.
For all these reasons this was one of the most unusual human events in history.