The birthday of the Prince of Wales is observed in the United Kingdom every June 21. This day is recognized as a designated flag flying day, during which the Union Flag is flown on all government buildings. The celebration marks the birthday of William, who became the Prince of Wales following the ascension of his father, King Charles III, to the throne. The title itself has historical roots dating back to the 12th century, first held by Owain Gwynedd. Although it is a day of significance, it is not a public holiday, meaning it does not result in a non-working day unless it coincides with a weekend. The observance reflects the royal heritage and the ongoing traditions associated with the British monarchy.