Sinterklaasdag, or St. Nicholas Day. In the Dutch-speaking countries in particular, children will have set out a pair of traditional wooden clogs on the doorstep for Sinterklaas to come along and fill them with goodies.
St. Nicholas is regarded as the patron saint of the city of Amsterdam, as well as of children and sailors. He is a beloved personage who traces his roots to an actual historical figure -- Nicholas, Bishop of Myra (read my post on the role of Nicholas in the history of Santa Claus here).
In modern times, the celebration of Sinterklaas has lost its Christian significance (though Christmas has not for many) -- it is simply a time to celebrate and give gifts. In fact, more gifts are exchanged on Sinterklaas Day than at Christmas.
In Amsterdam, the day includes a parade where Sinterklaas appears -- supposedly from Spain -- with an entourage of mischievous, black-faced helpers called Zwarte Pieten, reminders of the Moorish influence in medieval Spain. In the Middle Ages, "Zwarte Piet" (meaning Black Pete) was a nickname for the devil; St. Nicholas, having triumphed over the devil, subjugated the little devils, so to speak, into his service.