As the longest reigning monarch in British history, the Queen is, of course, the first to reach such a momentous milestone.
It is expected she will spend the day privately at Sandringham, as she often does, prior to returning to London later in the week. As her former press secretary Dickie Arbiter, has remarked: "She has always made it clear that her long reign is a consequence of her father's early death and so it is not a day for celebration".
As is widely known, Princess Elizabeth became Queen in Kenya, quite possibly as she was sitting on a viewing platform photographing wildlife at Treetops Hotel in the Aberdare National Park (incidentally, the Treetops is still in business and very proud of its royal connection: you can walk in the Queen's footsteps at the lodge, with prices starting at an eminently reasonable 274 USD for a double room; other notable guests include Joan Crawford, Lord Mountbatten, Lord Baden-Powell, the founder of the Boy Scouts movement, and the Beatles).
The Treetops then and now: the original lodge, built in 1932, was burnt down during the Mau Mau uprising in 1954. However, it was rebuilt in the same location, allowing the Queen and the Duke of Edinburgh to make a repeat visit in November 1983 (below):
Princess Elizabeth and the Duke of Edinburgh were in Kenya on the first stage of a tour of the Commonwealth that was due to last months and take the couple to Australia and New Zealand as well. The King, by then gravely ill with lung cancer and other ailments, was unable to undertake long-haul travel but had seemed in good enough health in the days leading up to his daughter's departure.
In fact, on 30 January 1952, the night before the Princess and the Duke were due to fly off on their tour, the King had been well enough to go the Theatre Royal, Drury Lane, to attend a performance of South Pacific, a Rodgers & Hammerstein musical, with the rest of the family (including Princess Margaret and Peter Townsend):
The following morning, 31 January, despite his doctors' advice, the King was at London Airport (now Heathrow Airport) to see Elizabeth off. It was the last time he was seen in public:
The couple were based at Sagana Lodge on the foothills of Mount Kenya. The lodge had been a wedding present to Princess Elizabeth and the Duke of Edinburgh in 1947. It is now one of the official residences of the President of Kenya and used for entertaining and Government summits:
Princess Elizabeth walking through the grounds:
In the early hours of 6 February 1952, the King died suddenly in his sleep at Sandringham, at the same time as Elizabeth and Philip were perched up in the branches of a giant wild fig tree to watch the animals, having gone to spend the night at the Treetops Hotel specifically to admire the wildlife.
It is well known that news of the King's death took several hours to reach the new Queen. Sir Edward Ford, the late King's assistant private secretary, believed that this was due to the Palace telegram bearing the news being misplaced along the way: it is possible, Sir Edward speculated, that the telegraphist took the agreed code for the King's death, "Hyde Park Corner", as the address and not the message.
In the end, it was Martin Charteris, Princess Elizabeth's private secretary, who got wind of the news, thanks to a chance encounter with a journalist at the Outspan Hotel in Nyeri, where he had gone for lunch.
Charteris alerted Michael Parker, Prince Philip's private secretary, who in turn relayed the news to his boss. It was 2.45 pm local time, 11.45 am in London, when Princess Elizabeth was told she had become Queen.
There survives a photo of the new Queen walking in the grounds of Sagana Lodge with Kenya Governor Sir Philip Mitchell in those early hours after hearing of the death of her father, wearing a light-coloured patterned dress. That was because the majority of her tour wardrobe, including the mourning clothes Royals always travel with, had been sent ahead to Mombasa, to be embarked on the SS Gothic for New Zealand and Australia.
Therefore the Queen flew from Nanyuki to Entebbe, Uganda, where she was reunited with the rest of her clothes and was finally able to change. By mid-afternoon on the following day, the royal party was back on British soil. At Heathrow Airport to receive the Queen were her uncle the Duke of Gloucester, the Mountbattens, Prime Minister Winston Churchill and other senior politicians, and one of the Duke of Edinburgh's equerries, Beresford Horsley, who carried a note from Queen Mary to the new Queen.
It is among the most impressive brooches in the Queen's collection, measuring 8.2 x 3.8 cm. There are two more almost identical brooches of this design, presented to the Queen Mother and Princess Margaret in June 1953 in Bulawayo, on Queen Elizabeth's first Commonwealth tour as Queen Mother. Incidentally, you can also get a perfect replica for yourself from the Royal Collection Shop.
After the coronation, the Queen resumed the Commonwealth tour she had been forced to cut short due to the death of her father, and embarked on a mammoth 174-day tour from November 1953 to May 1954, touching Bermuda, Jamaica, Fiji, Tonga, New Zealand, Australia (below, on board HMAS Australia in March 1954), the Cocos Islands, Ceylon, Aden, Uganda, Malta and Gibraltar:
Historic Royal Palaces has a section dedicated to the Sappire Jubilee on their website, with a selection of themed souvenirs. Will you be getting a memento for yourself? I'll admit my eye has been caught by those fab sapphire Christmas tree decorations!