The occasion will be marked by the ceremony of Trooping the Colour (King's Birthday Parade), the annual military spectacle from Horse Guards in London.
King Charles III's actual birthday is 14 November - he was born on 14 November 1948 - but the Sovereign's birthday (or HM The King's official birthday) is celebrated on another date, usually in the summer months.
Her Majesty Queen Elizabeth II's official birthday was celebrated in June, while her actual birthday was on 21 April.
The morning’s events culminate with the annual RAF fly-past and the royal family’s iconic balcony appearance.
All the best Jan