Kensington Palace, which oversees events for the Prince and his wife Kate Middleton, shared a photo of William at a black tie event, captioned: "Happy birthday to the Duke of Cambridge!"
Prince Louis will be christened next month.
Princess Charlotte was two-months-old.
Britain's royal palace says Prince Louis will be christened July 9 by Archbishop of Canterbury Justin Welby. His godparents will also be announced at the event.
Members of the Royal Family will gather at the chapel to celebrate the christening of their youngest member, Louis.
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However, they failed to complete each of those opportunities and that is where Portugal got there job done early in the game. When they managed to stop appealing and resumed play, Morocco looked risky and were the dominant side in the second half.
Louis was born at the Lindo Wing of St Mary's Hospital in London's Paddington, on St George's Day. Charlotte, 3, was christened at Sandringham's St. Mary Magdalene church in Norfolk, the same place where her grandmother, Princess Diana, was baptized.
Newlyweds the Duke and Duchess of Sussex are likely to attend the family celebration.
Louis will be wearing the same cream lace and satin gown worn by George and Charlotte.
It's the quickest christening out of all of the Duke and Duchess' children.
Guests can expect to chow down on a slice of William and Kate's wedding cake, which was a fruit cake envisioned by Fiona Cairns.