Every summer the Mall outside Buckingham Palace is filled with flag-waving, Union Jack-clad merrymakers, all gathered to catch a glimpse of the Royal Family as they wave to crowds on the balcony for the Queen's birthday.
This year, the world will be hoping to catch a glimpse of the Duke and Duchess of Sussex's baby boy or girl at the Trooping the Colour celebrations.
But as Elizabeth II wasn't actually born in June, why do the family celebrate her birth then? Here is everything you need to know about the Queen's birthdays – that's right, both her real and official celebrations.
When is the Queen's birthday?
The Queen was born on April 21 1926; this year she will turn 93.
However her birthday is officially celebrated in June every year. This celebration, referred to as the Trooping the Colour, is a moveable feast – in 2019 it is celebrated on June 8.
How does the Queen celebrate her birthday in April?
The Queen usually celebrates her real birthday in private with her family. While the fanfare is mostly saved for her official birthday in the summer, every year on April 21 there are several gun salutes in London at midday.
What does 'Trooping the Colour' mean?
Acting as the personal bodyguards of the Queen, the Guards are one of the oldest regiments of the British Army. They have been a constant fixture of the monarchy since the English Civil War ended in 1660.
'Colours' were the regimental flags of the British Army which displayed the uniform colours and insignia of different units. They were designed to help troops quickly identify their unit on the battlefield and avoid confusion.
In order for troops to be familiar with their regiment's Colours, it was necessary to display them regularly. So, young officers would march in between the ranks of troops stood in lines holding the Colours high.
This is where the word 'trooping' comes from. The Colour of the troops refers to the historical colour-coding of British regiments worn on their uniforms and represented in each regiment's flag.
Why does the Queen have two birthdays?
Although the trooping of the colours was first performed for military purposes under King Charles II in the 1600s, the parade became an official part of the British calendar a century later.
It is a tradition that was started by George II in 1748 and it owes its origins to the ageless problem of the British weather.
George was born in November and felt the weather would be too cold at that time of year for a birthday parade. King George decided to combine his birthday celebration with an annual military parade.
It is a tradition that has continued to this day. All British sovereigns are given the option of having an 'official' birthday and, because the Queen's real birthday is on April 21, she chose to to hold her celebration in June each year.
When she first ascended the throne, the Queen chose to hold her Official Birthday on the second Thursday of June; this was the day her father, King George VI, chose to celebrate his official birthday.
However in 1959 the Queen decided her official birthday should be held two days later, on the second Saturday of June, instead – and it has been ever since.
What happens during Trooping the Colour?
Otherwise known as The Queen's Birthday Parade, the Queen inspects soldiers from the Household Division; it takes place on Horse Guards Parade behind Whitehall.
The colourful display of pageantry features 1400 officers and men on parade, 200 horses and 400 musicians from 10 bands. The Queen always attends and takes the salute.
According to the Household Division, 113 words of command are given by the Officer in Command of the parade and each year a different regiment's colours are trooped.
It begins when the Queen leaves Buckingham Palace in a carriage, accompanied by a Sovereign's escort from the Household Cavalry Mounted Regiment at 10am.
She used to arrive riding side-saddle on a horse, wearing the uniform of the regiment begin trooped; however since 1987 she has arrived by carriage.
She arrives at Horse Guards Parade to take the Royal salute from the officers and men on parade at 11am and then carries out an inspection of the troops wearing the ceremonial uniform of red tunics and bearskin hats.
The Regimental Colour (flag) being trooped is then carried down the ranks following music by the bands.
Once the soldiers have marched past the Queen, she returns to Buckingham Palace for a second salute.
There, she is joined by members of the Royal Family on the balcony of the palace. Following this, a 41-gun salute is conducted in Green Park.
The Queen has taken the salute at every parade since her accession to the throne 67 years ago, except in 1955 when there was a national rail strike.
How can I watch it?
The Queen's Birthday Parade is televised live by the BBC each year.
While it is possible to watch the parade up close, it is unfortunately too late to get tickets now. The ballot for seated tickets around the Horse Guards closed at the beginning of March, while standing tickets sold out following this.
However, the Queen and other members of the Royal family travel to and from Buckingham Palace in carriages, so there is ample opportunity to see her for anyone standing on The Mall.
The public can also admire the spectacle during the two rehearsals. The first, known as The Major General's Review, will take place on Saturday May 25; The Colonel’s Review will take place on Saturday June 1. Both can be seen from The Mall or the edge of St James's Park.