"Kate's not into big birthday celebrations, but [Prince] George and [Princess] Charlotte adore birthday cake, so there will be a special tea party at the Palace with candles and presents", an insider told the publication's royal reporter Katie Nicholl.
Other pics in the brief slideshow include Kate having a laugh in the woods and having miniature Union Jacks poked in her face by some children.
The royal dad visited the Royal London Hospital and was announced as the patron of the air ambulance's anniversary campaign on Wednesday, which happens to be his wife Kate Middleton's 37th birthday.
The group gave William a card for his wife's birthday, which was made by a group of children, according to the U.K.'s Hello magazine.
Today, her husband Prince William was out and about for the first time since he and Kate spent Christmas with Queen Elizabeth II on the Sandringham Estate. "I did remember this morning, so that was ok", he said.
Proud father Prince William revealed Prince Louis is teething.
He was at the official visit for just one hour, thought to have rushed away to celebrate the Duchess of Cambridge's birthday with their three children.
Kensington Palace took to their Twitter account to share a touching tribute of the royal, tweeting: "Thank you everyone for all your lovely messages on The Duchess of Cambridge's birthday", along with a balloon emoji.
George will be celebrating his sixth birthday in July, while baby Louis, turns one on April 23.
She graduated in 2005 with a 2:1 in History of Art from St Andrews University where she met and began dating Prince William, whom she married in 2011.
The couple joined Kate's parents, her brother James, and sister Pippa, to see a Cirque de Soleil show at the Royal Albert Hall in 2013.