The traditional Danish New Year's Eve custom of breaking old plates against friends' doors is called plate smashing.
In this custom, people break unwanted dishes outside the homes of friends or relatives. A pile of broken plates was traditionally interpreted as a sign that the household had many loyal friends, because more people had visited to leave the pieces. The practice is associated with luck, friendship, and a noisy welcome to the new year.
The custom is often discussed alongside other Danish New Year's traditions, including jumping from a chair or sofa at midnight to symbolically leap into the new year. Speeches, fireworks, and the dessert-like New Year's cake called kransekage are also part of Danish celebrations.
“Plate smashing” describes the distinctive tradition; it does not mean smashing plates during an official public ceremony. Accounts also note that the practice has become less common in modern Denmark, partly because of safety, mess, and changing household habits.