The first modern European royal to stake a claim to the date was Queen Letizia of Spain who was born on September 15th 1972 in Oviedo. Her arrival was a cause of huge celebration for her parents, Jose Ortiz and Paloma Rocasolano, but it became a famous day in Spanish royal history when Letizia married Felipe of Spain, then Prince of Asturias, on May 22nd 2004 and became the first non royal in line to be her country's queen. Letizia became consort on June 19th 2014 on the accession of her husband as King Felipe VI - turning September 15th into the birthday of a queen.
It was already the beginning of a king. On September 15th 1973, King Carl XVI Gustaf took the throne of Sweden on the death of his grandfather, King Gustaf VI Adolf, after a reign lasting almost 23 years. The rule of Carl Gustaf has seen many changes - the rules about royal marriage were altered to let him marry someone of non royal birth while the succession laws have also been transformed meaning his first child, Victoria, is his heir. On September 15th 2017, Carl XVI Gustaf will celebrate 44 years as King of Sweden. That's about as royal as you get.
On the very same day that Carl Gustaf took his country's throne, a baby boy was born in Orebro in central Sweden to Olle and Ewa Westling. Baby Olof Daniel would grow up to marry the heir of the man who became Swedish king that day. The wedding of Daniel Westling and Crown Princess Victoria of Sweden in 2010 was the biggest Europe had seen since that of the Prince of Wales and Lady Diana Spencer in 1981. The couple has since had two children - Estelle, born in 2012, and Oscar, born on March 2nd 2016. September 15th has become a special day for this part of the Swedish royal dynasty.