It was a happy day for many in Spain, 13 September 1961, because the engagement was announced between Prince Juan Carlos de Borbón and Princess Sophia, the daughter of King Paul of Greece and Frederica of Hanover. The couple and their families were photographed outside the home of Queen Victoria Eugenie, the wife of King Alfonso XIII, in Lausanne Switzerland.
The couple had known each other for several years. They met when Sophia's mother organised a Mediterranean cruise on a yacht in 1954 and invited several young, eligible royals along. Four years later, there were rumours of a romance.
Juan Carlos spent much of the summer of 1961 at the holiday home of the Greek royal family, but their own families denied the couple were in love, and it is believed the family of Juan Carlos in particular wanted to keep it secret from General Franco because their relationship with the dictator was not as good as it had previously been.
Sofía (as her name is spelled in Spain) has often joked that her prince never actually proposed to her. He is reported to have tossed a small box at her during an event at a hotel in Lausanne, and said "Sofía, catch it". When she opened it and found a ring made from melted coins dating back to the time of Alexander the Great, he said something like "So we'll get married now, then."
The couple married on 14 May the following year, and there were two ceremonies in Athens, Greece: the first was a Roman Catholic ceremony at the Cathedral of Saint Denis, and then a Greek Orthodox ceremony at the Metropolitan Orthodox Cathedral of the Virgin Mary.
Juan Carlos and Sofía had three children: the eldest was the Infanta Elena, now the Duchess of Lugo, who was born on 20 December 1963, followed by the Infanta Cristina Federica, born on 13 June 1965, and Felipe, the present king following his father's abdication in 2014, who was born on 30 January 1968.
Juan Carlos ascended to the throne in 1975, following the death of Franco.
Has it been a happy marriage? It appears to have been turbulent at times and for years there were rumours of the king's adultery. In recent years there have been stories of the couple leading increasingly separate lives, including inhabiting different wings of the palace, especially after Juan Carlos stepped down in favour of Felipe.
Now, however, some writers for the gossip press say there has been a reconciliation and the King and Queen Emeritus are closer than they have been for years. Who knows? But on that day in 1961, their smiles were genuinely happy and they appeared to be very much in love and looking forward to a future together.