The tradition of nytårsløjer goes back a long way. 

In the Middle Ages, people in Denmark believed that evil spirits were at their mischievous best during the shift from the old year to the new.

To fend off these troublesome spirits, people embraced a variety of customs, one of which involved tossing pottery at each other’s doorsteps. This was a way of shooing away any lingering negativity and starting the New Year with a clean slate.

In the early 19th century, when farmers who had in some way upset others in their villages were likely to come home to find their cart hauled up onto the roof of their house. 

Else Marie Kofod, head of the Folklore Archives at the Royal Danish Library, told the Jyllands-Posten newspaper that she believed this sort of New Year’s tomfoolery is becoming less common. 

“To make fun of each other like that requires that the perpetrator and recipient know each other. But today, many people don’t know their neighbours very well, so what the perpetrator perceives as a funny New Year’s prank could easily be perceived as vandalism by the recipient,” she said. 

But nytårsløjer still happen, with local newspapers every year documenting some of the wilder and more imaginative tricks played in their area. During last year’s celebrations, for example, a man in Herning in central Jutland awoke to find that ten of his neighbours’ wheelie bins had been parked in his drive. 

The perennial classics: 

• Hoisting a Christmas tree or other incongruous object up a neighbour’s flagpole is the New Year’s prank most beloved of Danes, perhaps because it involves flags. You can see a photo of it in the TV2 story about the wheelie bins. 
• Ringing a doorbell and covering the person who answers with confetti. This is something you might do to a neighbour or friend you know well.
• Wrapping trees, bicycles, postboxes, or indeed anything else in toilet paper or clingfilm. If you drive around a Danish town or village on New Year’s Day, you are likely to see some evidence of this. 
• Writing “Godt Nytår!” on windows and walls with silly string or washable spray paint.
• Putting sellotape or gaffer tape over someone’s keyhole.
• Putting bangers between the toilet seat and the toilet bowl. This is done using the bangers that explode on impact when thrown to the ground. The hope is that they will detonate when someone sits on the toilet. 

Pranks closer to vandalism: 

The Danish insurance company Alm. Brand Group said in a press release last year that it has seen a rise in insurance claims related to New Year’s Eve attacks on post boxes and wheelie bins.

It recommends locking both if possible, putting a water-filled tray in post boxes to prevent them catching fire and moving bins away from the road. 

Spraying shaving foam on windows, doors and cars may seem like relatively harmless fun but shaving foam contains chemicals that can damage the paintwork of cars and buildings, so this is also a prank best avoided.  

Finally, while throwing eggs at walls and doors, or firing them with slingshots, may seem almost old-fashioned, eggs can leave stains that are hard to remove, so you are likely to ruin someone’s day.