or Santa Klaus. A corrupt contraction of Sankt Nikolaus (Sankni kolaus - i.e. St. Nicolas), the patron saint of children. The vigil of his feast is still held in some places, but for the most part his name is now associated with Christmas-tide. The old custom used to be for someone, on December 5th, to assume the costume of a bishop and distribute small gifts to “good children.” The present custom is to put toys and other little presents into a stocking or pillow-case late on Christmas Eve, when the children are asleep, and when they wake on Christmas morn each child finds in the stocking or bag hung at the bedside the gift sent by Santa Claus. St. Nicholas' day is December 6. The Dutch Kriss Kringle.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894