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

On Monday,Prince Williamwore 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 theOrder of the Garter paradeand service atSt. George’s Chapel, Windsor — the scene ofthe magnificent weddingofPrince HarryandMeghan Markleon May 19.

William, his fatherPrince Charles, 69, and other senior royals joinedQueen Elizabethfor 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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Britain Royals Order of The Garter, Windsor, United Kingdom - 18 Jun 2018

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Order Of The Garter Service

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Order Of The Garter Service

Kate Middleton, who is on maternity leave since giving birth toPrince Louisin April, was not at the annual event.

William wasmade a member of the orderin 2008, two years before he got engaged toKate Middleton.

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

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Order of the Garter service, St George’s Chapel, Windsor Castle, UK - 18 Jun 2018

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Order Of The Garter Service

Order of the Garter service, St George’s Chapel, Windsor Castle, UK - 18 Jun 2018

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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.

source: people.com