"The Queen celebrates two birthdays each year: her actual birthday on 21 April and her official birthday on a Saturday in June.
Official celebrations to mark Sovereigns' birthday have often been held on a day other than the actual birthday, particularly when the actual birthday has not been in the summer. King Edward VII, for example, 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.
The Queen usually spends her actual birthday privately, but the occasion is marked publicly by gun salutes in central London at midday: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park and a 62 gun salute at the Tower of London. In 2006, Her Majesty celebrated her 80th Birthday in 2006 with a walkabout in the streets outside of Windsor Castle to meet well-wishers.
On her official birthday, Her Majesty is joined by other members of the Royal Family at the spectacular Trooping the Colour parade which moves between Buckingham Palace, The Mall and Horseguards’ Parade."
The Duke of Cambridge, Prince of Wales, Duke of Kent, and the Princess Royal
"Since 1748, this parade has also marked the Sovereign's official birthday. From the reign of Edward VII onwards, the Sovereign has taken the salute in person at Trooping the Colour.
During the ceremony, The Queen is greeted by a Royal salute and carries out an inspection of the troops.
After the massed bands have performed a musical 'troop', the escorted Regimental Colour is carried down the ranks.
The Foot Guards and the Household Cavalry then march past Her Majesty, and The King's Troop, Royal Horse Artillery, rank past.
The Queen rides in a carriage back to Buckingham Palace at the head of her Guards, before taking the salute at the Palace from a dais. The troops then return to barracks.
Her Majesty then joins other members of the Royal Family on the palace balcony for a fly-past by the Royal Air Force.
The Queen has attended Trooping the Colour every year of her reign, except in 1955 when a national rail strike cancelled the event."
As with many formal traditions, Trooping the Colour emerged from something practical - in this case, the need for foot soldiers to be able to visually identify the flags of their battalions as rallying points above the chaos of battle."The aim of the ceremony was to familiarise each man with the coloured flags that identified his unit, and to guarantee all ranks would recognise their assembly point, especially when stationed in an unfamiliar town.Each morning, the colours were escorted from the billet back to their position in the battalion ranks. Consequently, the colours came to express the spirit of the regiment and were held in the highest regard.
In time the Regimental Colour has taken on a greater significance. Its folds of emboidered cloth are an important object of reverence and a memorial to lost comrades."
Of course, this year's Birthday Parade followed the day immediately after Prince Philip's 90th birthday and so must have been an especially momentous family event. Happy and glorious, indeed. God Save the Queen!
Read more from the official website of The British Monarchy here, from The Daily Mail here, and from British Military Ceremonial here.