Kate Middleton, Prince William, Prince Harry and the rest of the royal family celebrated Queen Elizabeth II turning 88 at a Trooping the Colour parade in London on Saturday, June 14

Let the royal festivities commence! Kate Middleton, along with the rest of the royal family, celebrated Queen Elizabeth II's 88th birthday in London on Saturday, June 14.

The family honored the Queen with a Trooping the Colour parade, and were then escorted to the balcony of the Buckingham Palace. She was joined by husband Prince Philip, Duke of Edinburgh, Duchess Kate, Prince William, Prince Harry, son Prince Charles and his wife Camila, the Duchess of Cornwall.

The Queen looked appropriately regal in a blue satin silk coat and a white hat with a navy trim. She smiled as she waved to the thousands of Brits standing below. Prince Harry, meanwhile, shared several laughs with sister-in-law Middleton, 32, and older brother William, 31.

Middleton, who often recycles her outfits like her royal in-laws, wore a white coat dress by Alexander McQueen that she's never worn before. She kept her tousled brunette locks down, and accessorized with a matching Jane Taylor Milinery hat.

Middleton's siblings have a busy day ahead of them, too. Pippa Middleton, 30, and James Middleton, 27, are set to take part in Race Across America, a 3,000-mile journey from Oceanside, Calif., to Annapolis, MD. On Friday, Pippa was also named the new ambassador for the British Heart Foundation.