The truce occurred five months after hostilities had begun. Lulls occurred in the fighting as armies ran out of men and munitions and commanders reconsidered their strategies following the stalemate of the Race to the Sea and the indecisive result of the First Battle of Ypres. In the week leading up to 25 December, French, German, and British soldiers crossed trenches to exchange seasonal greetings and talk. In some areas, men from both sides ventured into no man’s land on Christmas Eve and Christmas Day to mingle and exchange food and souvenirs. There were joint burial ceremonies and prisoner swaps, while several meetings ended in carolling.
Many accounts of the truce involve one or more football matches played in no man’s land. This was mentioned in some of the earliest reports, with a letter written by a doctor attached to the Rifle Brigade, published in The Times on 1 January 1915, reporting “a football match… played between them and us in front of the trench”
https://en.wikipedia.org/wiki/Christmas_truce