A very ancient rule means that we Brits celebrate King Charles III's birthday in June, even though he was born in November.
Charles, who was born back in 1948, turned 77 late last year (14 November), but he'll be getting another birthday this Saturday (13 June).
This weekend will actually be the ruling monarch's 'official birthday', which falls on the second Saturday of June, which is when the UK will properly celebrate the occasion.
Charles' 'official birthday' also coincides with Trooping The Colour, which is a centuries-old tradition.
Like all other Kings and Queens of England, Charles' late mother, Queen Elizabeth II, was also granted the same honour and celebrated both her actual birthday, 21st April, as well as the June date.
King Charles III will celebrate his 'official birthday' this weekend despite being born in November (Pool / Pool / Getty Images) But why does Charles get two birthdays a year?
Well, simply put, it's actually all down to the weather.
The reason is simply that summer in England is the best time for a parade.
Essentially, because the monarch's birthday is typically celebrated with outdoor events, such as Trooping the Colour, it makes a lot more sense to celebrate in the summer than during the autumn or winter.
Royal Museums Greenwich notes: "With a November birthday being too cold for a celebratory parade, he tied his celebrations in with the annual Trooping the Colour military parade."
The two-birthday tradition for British monarchs dates all the way back to 1748, when King George II combined the annual summer military march with his own birthday celebration, although he was born in November.
The two-birthday tradition for British monarchs dates all the way back to 1748 (Karwai Tang / Contributor / Getty Images) Queen Elizabeth II’s great-grandfather, Edward VII, was also born in November, and so his birthday was celebrated in May or June throughout his reign.
The choice stuck, and Trooping the Colour has marked the official birthday of the British King or Queen for more than 260 years.
Also known as The King's Birthday Parade, Trooping the Colour is the annual military parade that features over 1,400 parading soldiers, 200 horses and 400 musicians.
The official Royal Family website outlines: "The streets are lined with crowds waving flags as the parade moves from Buckingham Palace and down The Mall to Horse Guards' Parade, alongside Members of the Royal Family on horseback and in carriages.
"The display closes with an RAF fly-past, watched by Members of the Royal Family from the Buckingham Palace balcony."
The name comes from the practice of 'trooping' the regiment's colours (the flag) down the ranks so soldiers could easily recognise their rallying point in the chaos of battle, per the British Army.