Which European country has a national holiday called King's Day, during which people dress up in orange to celebrate?

The national holiday known as King's Day is celebrated in the Netherlands on April 27, which marks the birthday of King Willem-Alexander. This day is notable for its vibrant and festive atmosphere, where citizens and visitors alike don orange attire, a color that symbolizes the Dutch royal family, which is from the House of Orange-Nassau. The celebration includes various activities such as street markets, music performances, and parties throughout the country.

In contrast, while Belgium has a king, it does not celebrate a similar holiday that features dressing in orange. Norway celebrates Constitution Day, and Denmark has its own national celebrations, but neither features an event akin to King's Day in terms of attire or national significance connected to the royal family. Therefore, the association of orange clothing specifically with a national celebration distinctly points to the Netherlands as the correct answer.