Thousands of people will flock to London to watch the Trooping the Colour at Buckingham Palace on Saturday to mark Queen Elizabeth II's official 90th birthday, with millions more expected to watch the annual celebration from home.
For decades, this tradition has been performed by British Army regiments on Horse Guards Parade to mark the monarch's birthday.
Here we tell you everything you need to know about the Trooping the Colour and what it means.
What is it?
Trooping the Colour is an annual ceremony performed by regiments of the British Army in June. It marks the official birthday of the monarch and is held in London on a Saturday on Horse Guards Parade.
The Queen's Colour of a battalion of Foot Guards is 'trooped', which means carried along the ranks, before the monarch. Only one colour can be trooped at a time, according to the British Army.
The five Household Regiments - Grenadier, Coldstream, Scots, Irish and Welsh Guards take their turn each year.
Who attends Trooping the Colour?
The Queen now attends the ceremony in a carriage, rather than riding side-saddle, which she did up until 1987.
More than 1,400 officers are on parade, as well as 200 horses. Hundreds of musicians are also in attendance along with the rest of the Royal family.
The parade route goes from Buckingham Palace to the The Mall to Horse Guards Parade to Whitehall and back again.
At 11am the Royal Procession arrives and the Queen takes the Royal salute.
The Queen inspects the troops, as her carriage slowly drives down the ranks of all the guards. After the bands have performed a musical 'troop' in slow and quick time, the Regimental Colour is escorted down the ranks of the guards.
The Foot Guards and the Household Cavalry then march past the Queen and The King's Troop, Royal Horse Artillery, rank past with their Guns.
When the event is over, members of the Royal family gather on the balcony of Buckingham Palace for a flypast by the Royal Air Force at 1pm.
When did the tradition start?
Trooping the Colour is a tradition that dates back to the 17th Century when the colours of the regiment were used as a rallying point in battle.
Since 1748, it also marks the official birthday of the monarch.
Today's ceremony derives from two old military traditions, Trooping the Colour and Mounting The Queen's Guard. Since George IV ascended the throne, the parade became an annual event, with the exception of two World Wars and a national rail strike in 1955.
How can you watch it?
Seated tickets for stands around Horse Guards Parade, which are allocated by ballot, are now sold out.
Those wishing to attend the event are advised to stand on The Mall or on the edge of St James's Park overlooking Horse Guards from 9.00am.
The ceremony begins at 10am and the flypast will take place at 1pm.
The parade will also be broadcast live on the BBC.