For Prince William though, royal duty called.
The Duke of Cambridge had a morning tea with this new patronage, the London's Air Ambulance charity. The Duke, a former air ambulance pilot himself, was recently named a patron of the charity's 30th anniversary campaign.
But, Wills did manage to sneak in a tiny reference to his wife.
When he was presented with a card for his wife during his visit today, he replied, "Well done for remembering," then added, "I did remember this morning, so that was okay!" according to People.
The response prompted the room to giggle.
Kate's last big birthday celebration was over six years ago, when the Duchess turned 31.
But since George's birth, Kate has celebrated her birthday privately in the comfort of her home.
A source told E! News that last year Kate spent her 36th birthday "privately with William and the children. It's just a quiet celebration at home. No big party."
This article originally appeared on marie claire Australia.