The King celebrated his 76th birthday on Thursday, stepping out to open a Coronation Food Hub in Southeast London.
But given that a birthday parade for the monarch took place in June, formally known as Trooping the Colour, some people may wonder why Charles marks his birthday twice?
It's all down to a tradition which dates way back to the 18th century, with the monarch having an official birthday (Trooping the Colour) and an unofficial birthday (in Charles case – 14 November).
Official celebrations to mark the sovereigns' birthday have often been held on a day other than their actual birthday, particularly when the day has not fallen on a day in the summer.
King Edward VII was born on 9 November, but his official birthday was marked throughout his reign in May or June when there was a greater likelihood of good weather for the birthday parade, also known as Trooping the Colour.
Similarly, the late Queen Elizabeth II's birthday was 21 April, but she always celebrated it officially on a Saturday in June.
What happens at Trooping the Colour?
Trooping the Colour has marked the monarch's official birthday for over 260 years, and this year marked Charles's second appearance at the event as King.
Over 1,400 parading soldiers, 200 horses and 400 musicians take part in the display of military precision, horsemanship and fanfare. The parade travels from Buckingham Palace down The Mall to Horse Guards Parade, and closes with a Royal Air Force fly-past.
This year, amid the King's cancer treatment, he travelled by carriage with the Queen to review the soldiers on parade. During the ceremony, which took place on Horse Guards Parade in Whitehall, he carried out the review of the Guardsmen and officers from an Ascot Landau carriage with the Queen.