Every second weekend of May, more than 600 wind and water mills across the Netherlands open their doors for National Mill Day. Many mills are normally inaccessible, so this is a rare chance to go inside, climb to the top, and watch the machinery in motion. Millers in traditional dress operate the sails, and you can often buy freshly ground flour. The event has been running since 1972 and is a great excuse to explore the Dutch countryside. Look up participating mills near you on the De Hollandsche Molen website.