Leaving cookies and milk for Santa Claus is a Christmas Eve tradition. Children regularly place a dish of cookies and a glass of milk by the tree on Christmas Eve so that Santa has enough energy to complete all of his gift-giving before kids wake up on Christmas morning. The tradition of leaving cookies out on Christmas Eve may trace its origins to Saint Nicholas. On the feast of St. Nicholas on December 6, the children would leave food and drink for the saint and any attendants. The treats would be exchanged for gifts.
While that’s one theory behind the tradition, others, think it is linked to the history of the Christmas tree and the edible decorations that adorn it. Santa would snack on the ornaments. Even after edible decorations gave way to inedible ornaments the tradition of leaving a snack for Santa remained.
Whatever the origins behind the tradition, millions of children dutifully leave milk and cookies for Santa on Christmas Eve.