Almost 20-years after the paparazzi-induced death of William's late mother, the prince released a statement via Clarence House encouraging the press to leave his girlfriend alone.
While Prince William was paying a visit to London's Air Ambulance Charity on Wednesday (Jan. 9), Kate was busy celebrating at home, likely with her three children - Prince George, 5, Princess Charlotte, 3, and eight-month-old Prince Louis.
They posted a photo of Kate during her visit to Cyprus in December 2018 with a short message: "Thank you everyone for all your lovely messages on The Duchess of Cambridge's birthday".
After the royal unveiled a handsome plaque to commemorate the charity's 30 anniversary campaign - of which William is a new patron - two adorable kids presented him with some gifts.
Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge (aka Kate Middleton) stroll towards the Church of St Mary Magdalene on the Sandringham estate on Sunday (January 6) in King's Lynn, England.
Kate received other birthday wishes from around the globe, with royal households, British embassies and charities which she supports as patron posting their congratulations.
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She will no doubt be spending most of the day with Prince Louis, who was born in last April.
Prince William also praised the children, who are brother and sister, for their handwriting.
William worked as an air ambulance pilot before retiring in 2017 to become a full-time royal.
"I couldn't go back without a toy helicopter". They celebrated with the Middleton family at their home (and William even spent the night!). William - a former search and rescue helicopter pilot in the Royal Air Force - will work as a co-pilot transporting patients to the hospital in emergency situations.
Meanwhile, Kate's first royal engagement of the year hasn't yet been revealed.
She will probably spend the day at Kensington Palace.