The Danish New Year's Eve tradition of smashing old plates against friends' doors is called plate smashing.
In the custom, people throw unwanted plates or dishes at the doors of friends and neighbors around New Year. A large pile of broken crockery outside a home is traditionally interpreted as evidence that the household has many friends. The noise and deliberate breakage also turn discarded household objects into a festive part of the celebration.
The practice belongs to a wider Danish New Year tradition that includes fireworks, festive meals, and the midnight dessert kransekage. Some modern accounts describe people using old plates saved for the purpose, while others note that the custom has become less common because of safety, cleanup, and changing household habits.
It is not the same as breaking a plate accidentally for luck, nor is it a universal Scandinavian custom. Denmark’s distinctive version is specifically associated with throwing crockery at the doors of people one knows.