He was named patron of the London Air Ambulance Service's anniversary campaign.
Prince William didn't need any reminders for today's special significance.
But, luckily for William the London Air Ambulance staff had a colourful toy helicopter ready for the royal tot.
Kate has had a busy 12 months on the family front, giving birth to Prince Louis last April and becoming a mother of three.
"Through his work as an air ambulance pilot for the East Anglian Air Ambulance, The Duke has seen first-hand the impact these first responders have upon the lives of patients with critical injuries", the palace said in a Wednesday statement.
Charlotte goes to Willcocks Nursery School, while George goes to Thomas's London Day School in Battersea.
When someone asked what the royal would be doing to mark Kate's big day, he jokingly replied, "That would be telling!"
William also served almost five years as a helicopter pilot with the RAF's Search and Rescue Force.
2009: Though 27 isn't exactly a milestone, it was a major birthday for Kate, marking the first in years that William was present for. There's no doubt that the future King will dote on his wife, too.
"She'd already had it the week before at Kensington Palace, as ludicrous as that might have been", the expert continued.