The Princess of Wales is celebrating her birthday with Prince William, Prince George, Princess Charlotte and Prince Louis.
Princess Kate, who turns 42 today, is expected to spend her birthday in Windsor alongside her husband and children.
A source told the Mail: "They'll be at Windsor as the children are just about to start back at school, so it will just be family.
"But to be honest, that's just the way she likes it."
The Princess of Wales is celebrating her birthday with Prince William, Prince George, Princess Charlotte and Prince Louis.
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It comes as the Princess of Wales looks set to receive a promotion in 2024.
King Charles is reportedly considering appointing Kate as a Royal Lady of the Order of the Garter, the most senior Order of Chivalry in Britain.
The monarch personally chooses the members with key royals such as Prince William, Queen Camilla, Princess Anne and Prince Edward already part of the Order of the Garter.
Royal commentator Richard Fitzwilliams backed the mooted promotion for the Princess of Wales.
Princess Kate, who turns 42 today, is expected to spend her birthday in Windsor alongside her husband and children.
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Fitzwilliams said: "It is a wonderful idea if he does decide to do it.
"This will be something she really would value because it is the premier order of chivalry."
The King may also give the Princess of Wales the power to grant royal warrants, alongside Queen Camilla and Prince William.
The monarch decides who grants royal warrants, Prince Philip and Charles when he held the title of the Prince of Wales had the power alongside Queen Elizabeth II.
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The King may also give the Princess of Wales the power to grant royal warrants.
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Currently, just King Charles has the power but he may grant Queen Camilla and Prince William similar roles to the ones held by Prince Philip and himself under the late Queen.
Some existing warrant holders told the Mail that having Kate officially endorse them would be a "huge boost to the British industry".
One said: "Interest would go through the roof, particularly in areas such as fashion and lifestyle."