Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge
Carl Court/WPA Pool/Getty
06/12/2011 AT 12:30 PM EDT
They haven't been this busy since their wedding day
The Duke and Duchess of Cambridge spent the morning with family on Sunday to celebrate the 90th birthday of his grandfather, Prince Philip, Duke of Edinburgh.
Stepping out of a vintage Rolls Royce in a blue brocade frock coat and her trusty, nude LK Bennett heels, Catherine made the short walk through the rain to the side entrance of St. George's Chapel, where a service was held in honor of Philip's service to Great Britain and the Queen.
Prince William was by her side, holding an umbrella over Catherine's head until they reached the entrance, where they mingled with Prince Harry and other guests, including the Queen, who wore a sky blue royal crepe coat with white silk print dress and matching Angela Kelly hat.
The event was the third in a whirlwind weekend for Catherine, who rode down the Mall in London in a horse-drawn carriage for the "Trooping of the Colour"
on Saturday, before attending the wedding of a friend
that afternoon with her sister Pippa Middleton.
After the service at St. George's chapel, Catherine joined 750 guests for a canapé-and-drinks reception at the Grand Reception room of St. George's Hall on the grounds of Windsor Castle.
She and Prince William then ate at a private lunch for the British royal family and the Duke of Edinburgh's wider family, including King Constantine of Greece and royalty from Jordan and Germany.
Other members of the royal family in attendance were Prince Charles and his wife Camilla, the Duchess of Cornwall, William's uncles, the Duke of York and the Earl of Wessex, and his aunt, the Princess Royal. William's cousins Princesses Eugenie and Beatrice also attended, as did bride-to-be Zara Phillips