"Thank you everyone for all your lovely messages on The Duchess of Cambridge’s birthday," they said.
Meanwhile, Prince William was at a tea party celebrating 30 years of the London Air Ambulance.
The Duke of Cambridge met with clinicians, paramedics, pilots and crew to talk about the issues they're presented with when dealing with emergencies.
Kensington Palace shared footage of William's day out, writing: "The Duke of Cambridge, #30YearsSavingLives Patron, visited Royal London Hospital to learn more about the challenges @LDNairambface in their life saving work as a first responder in the capital."