Nearly everyone knows that Santa Claus—the obese, old gent who squeezes himself down the chimney every Christmas Eve—is the American alter ego of St. Nicholas. Slimmer and less overtly jolly, St. Nicholas roams about Western Europe showering children with presents on his traditional feast day of Dec. 6. In the Netherlands and parts of Germany, children expect a visit from a white-bearded, ecclesiastically garbed "Sinterklaas" (his Dutch name), who decides whether they have been naughty or nice before handing out treats from his sack.

Dutch and German immigrants brought St. Nicholas to America in the early 19th century, and he began a process of assimilation, trading in his bishop's miter and crosier for a fur-trimmed red suit and cap. The Santa...