The Garter Service is held every June on the Monday of Royal Ascot Week for the Order of the Garter. The Order is the most senior and oldest British Order of Chivalry and was founded by King Edward III in 1348. The Order doesn't technically do anything, but it is the highest level of chivalry that can be bestowed on a member of the public — be they royal or not. Membership is limited to the Sovereign, the Prince of Wales, and no more than 24 members, all of whom are appointed by the Queen. The Order also includes Royal Knights (for members of the royal family, like Prince Philip and Prince William) and Stranger Knights, for foreign dignitaries. Traditionally the knighthood was only given to male members of the aristocracy, but now the order is given to men and women of varied backgrounds — although they tend to lean heavily on the upper classes. New members are always announced on Saint George's Day (April 23).
During the Garter Service, the Order members walk through Windsor Castle while decked out in traditional blue velvet robes, or mantles, and black velvet hats with white feathers. They then attend a special religious service at Saint George's Chapel in the castle before emerging from the Great West Door of the Chapel to march back up to the Upper Ward of the castle.