The Queen will celebrate her official birthday with the Trooping the Colour parade.
Royal fans will be eager to see the Duke and Duchess of Sussex, who married exactly three weeks ago, joining other members of the royal family at the traditional summer spectacle.
Harry and Meghan are expected to ride in a carriage to the event, staged on Whitehall's Horse Guards Parade, and later join members of the monarchy on Buckingham Palace's balcony to watch the RAF flypast and acknowledge the crowds.
The Queen, who recently had a successful eye operation to remove a cataract, will watch the Trooping ceremony from a dais in Horse Guards and later inspect the lines of guardsmen in their scarlet tunics and bearskins.
The annual event will see more than 1,000 soldiers taking part in the traditional display of pomp and pageantry. The colour being paraded this year is the flag of the 1st Battalion Coldstream Guards.
The Duke of Edinburgh, who celebrates his 97th birthday on Sunday, has retired from official public duties and is not expected to attend.
The royal colonels, all on horseback, will accompany the Queen: Prince of Wales, Colonel of the Welsh Guards, the Princess Royal, Colonel of the Blues and Royals, and the Duke of Cambridge, Colonel of the Irish Guards.
Riding in the ceremony for the first time will be the Duke of York in his new role as Colonel of the Grenadier Guards.
As with previous years, the Duchess of Cambridge and the Duchess of Cornwall are expected to take part in the Queen's carriage procession.
The procession will be accompanied by a Sovereign's Escort of the Household Cavalry, made up of Life Guards and Blues and Royals, in their silver and gold breastplates and plumed helmets.
The massed bands of the Household Division will provide musical backing during the day and also taking part is the King's Troop Royal Horse Artillery who, following the parade, will fire a 41-gun salute in Green Park to mark the Queen's official birthday.
After the ceremony the royal family will head back to Buckingham Palace and gather on the balcony to watch the RAF flypast.
Among the 23 aircraft taking part are modern jets and historic aircraft, while the Red Arrows will be the finale.
Trooping the Colour originated from traditional preparations for battle. Colours, or flags, were carried, or "trooped", down the rank so that they could be seen and recognised by the soldiers.
In the 18th century, guards from the royal palaces assembled daily on Horse Guards to "troop the colours" and in 1748 it was announced that the parade would also mark the Sovereign's official birthday.
The Queen's actual birthday was on April 21 when she turned 92.