At MercatorNet, Michael Cook writes about St. Nicholas. Here’s how the tradition began:
[H]e was once regarded as the patron of unmarriageable girls. A nobleman in the town of Patara had three daughters. He had been reduced to such poverty that he could not provide them with dowries for their weddings and decided to sell them into prostitution to keep them from starvation. Nicholas heard of their plight and threw purses filled with gold into the house as each girl came of age.
From this incident arose the custom of giving children presents on his feastday, December 6, which is widespread in Europe.
Interesting beginnings for the plump, red-robed, toy-bearing character who today shows up at parades and malls.