It's been quite a day for Kate Middleton! Kate Middleton and Prince William attended the Queen's birthday celebration earlier today, and this afternoon the Duchess of Cambridge changed her dress, but kept her fascinator, to join her sister Pippa at their friend's wedding. Kate and Pippa were both in black pumps and summery dresses to watch jockey Sam Waley Cohen and his bride Bella Ballin tie the knot in Berkshire, UK. Princess Beatrice and her boyfriend Dave Clark also attended the ceremony. The groom reportedly played a crucial role in the relationship of Kate and Prince William, since Sam is credited with bringing the couple back together when they briefly split. Sam and Bella have been longtime friends of Kate and Will and were on the guest list for the royal wedding in April. Kate made her first official royal appearances this week as well, stepping out in a stunning Jenny Packham gown for the ARK dinner on Thursday.