Prince William Wears Velvet and Feathers for One of the Fanciest Days on the Royal Calendar!

Royals don’t do summer casual — at least not when they’re on duty.

On Monday, Prince William wore a traditional velvet robe and hat (complete with ostrich feathers!) for Garter Day, which celebrates the oldest and most senior order of knighthood in the U.K.

William, 35, took part in the Order of the Garter parade and service at St. George’s Chapel, Windsor — the scene of the magnificent wedding of Prince Harry and Meghan Markle on May 19.

William, his father Prince Charles, 69, and other senior royals joined Queen Elizabeth for the service, which last took place in 2016. Last year the service was canceled when the date of the State Opening of Parliament suddenly moved following a surprise general election.

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William proceeded down from the castle alongside his father. The Queen, who was without Prince Philip this year (he retired from his official royal duties last summer), arrived by car.

William was made a member of the order in 2008, two years before he got engaged to Kate Middleton.

After the half-hour service in the chapel, William joined the other royals, including Camilla, Duchess of Cornwall, in a carriage procession back up to the castle’s state apartments.

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Garter Day begins a week of festivities at Windsor, where the Queen is based as she heads to Royal Ascot horse racing on Tuesday and through the rest of the week.

Not present at the service: Harry and Meghan. Harry has sometimes attended to watch the proceedings but is not a Garter knight.