If you don't know, Duchess Kate is going to be celebrating her birthday in just a few days on January 9!
Kate, 36, wore a blue Catherine Walker coat which she was first seen in when pregnant with Prince Louis last February.
But, in a lighter moment, he was given a birthday card to pass on to his wife, the Duchess of Cambridge, who turned 37 on Wednesday.
Charlotte goes to Willcocks Nursery School, while George goes to Thomas's London Day School in Battersea.
The duke said "I did remember this morning, so I was OK", when given the card made by the young girl, whose family met the royal during his trip to the Royal London Hospital in east London.
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When someone asked what the royal would be doing to mark Kate's big day, he jokingly replied, "That would be telling!"
The charity, which delivers a trauma team to critically injured people in London, is close to William's heart.
William has an official engagement in the day, meeting pilots and paramedics from London's Air Ambulance. 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.
The Duke and Duchess of Sussex first appearance of the year will be on January 14.
Past year the heavily pregnant Duchess spent the day with her family at Kensington Palace.