After the Queen and Prince Charles and Camilla posted unique tributes on their Instagram pages, the Duke and Duchess of Sussex wished her a Happy Birthday in the comments section of Kate's "thank you" post.
It's believed to be taken in front of the Cambridge family's country home, Anmer Hall, last year.
Kate Middleton and Prince William Announce First Engagements of 2020 - Including a Palace Reception! When she was still alive, she was more popularly known as HRH Diana, Princess of Wales until her 1996 divorce from Prince Charles when she was stripped off her royal title.
Wearing a white and blue checked shirt underneath a beige sweater and jeans, Kate is wearing the same outfit as the one she wore for the family's 2018 Christmas card, alongside Prince William, Prince George, Princess Charlotte and Prince Louis.
She's reportedly owned the crew neck knit by Swedish label Fjallraven Ovik for a number of years, and while it's sold out - of course - we've found a few alternatives that we think would get the royal tick of approval.
Duchess Kate joined Bake Off star, Mary Berry in opening a family-friendly garden at Surrey's RHS Garden Wisely. If this story of betrayal is true it is probably false, or Kate is just very good at forgiving and dissimulating. And Kate Middleton in our opinion of Queen Elizabeth has a lot: her role has shaped him on the field after the 2011 royal wedding without ever slipping, one look too many, a word said at the wrong time.
Kate's parents Carol and Michael Middleton were among the guests.
She wrote: "While Kate may or may not be cracking open a bottle of champers over a boozy lunch, Prince William will be at work".
And maybe the arrival of a fourth child for Kate Middelton?