Traditional Dutch folklore about St. Nick includes his servant called Zwarte Piet, or "Black Pete". December 5th is Sinterklaas, the St. Nicholas feast day, and some pageants, parades, and parties include people dressing up in a Zwart Piet costume. The origin stories of Black Pete vary a bit, so the reason for his blackface depiction is either that he's a Spanish Moor or he is covered in soot from going down a chimney.
There's been debate in The Netherlands about whether or not Zwarte Piet's depiction is racist, but many Dutch see the character as a beloved part of a holiday tradition. Here are some Instagrams of this week's festivities: