The "vierdaagse" or "the Four Days Marches" are an international event hold each year in the 3rd week of July in Nijmegen, the Netherlands.
When I was a child we always went to watch. Now a days it is not as much fun watching the participants, but the city is transformed into one big party. Here you can see that they decorated the streets with walking socks (it also says walking socks "wandelsokken" in the red letters on the right). More info can be found obviously on wikipedia, but here is the link to the official site: https://www.4daagse.nl/en/
