St. Nicholas is the patron saint of Russia and Greece, of a number of cities, and of sailors and children, among many other groups, and was noted for his generosity.

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His legend of a kindly old man was united with old Nordic folktales of a magician who punished naughty children and rewarded good children with presents. The resulting image of Santa Claus in the United States crystallized in the 19th century, and he has ever since remained the patron of the gift-giving festival of Christmas.
St. Nicholas Day has remained a time when children are given special cookies, candies, and gifts. In many places, children leave letters for St. Nicholas.
Source: britannica.com