One of these days, Carl XVI Gustaf becomes the longest reigning monarch in Swedish history.
The previous record holder was Magnus Eriksson, who at the age of three was acclaimed King of Sweden on 8 July 1319. His father, Erik Magnusson, Duke of Södermanland, was the younger brother of King Birger Magnusson, who in 1317 imprisoned both his brothers (Erik and Valdemar). The brothers were either killed in captivity or left to starve to death. His brutal treatment of his brothers led to a rebellion, which forced King Birger to flee to Denmark and placed the young Magnus Eriksson on the Swedish throne.
He was already King of Norway, having inherited the Norwegian crown from his maternal grandfather two months previously, but in 1343 the personal union was dissolved when Magnus ceded Norway to his second son, Håkon VI, while his eldest son Erik was declared heir to Sweden.
In 1357, Erik became co-monarch of Sweden, but following his death two years later Magnus Eriksson was again the sole ruler of Sweden until February 1362, when Håkon was elected co-ruler. Magnus and Håkon were both deposed as Swedish kings in February 1364. As the exact date of this event is unknown, it is impossible to say for sure exactly how long Magnus’s reign lasted and therefore also to calculate on which date Carl XVI Gustaf overtakes him, but it is one of these days.
King Carl Gustaf came to the Swedish throne at the age of 27 when his grandfather, King Gustaf VI Adolf, died at 8.35 p.m. on 15 September 1973. (His father, Prince Gustaf Adolf, had died in a plane crash in 1947).
There will be no official celebrations of this milestone. King Carl Gustaf himself, accompanied by Queen Silvia, is on an official visit to Japan these days.
UPDATE: After consultations with the National Archives, the royal court chose to mark the occasion on 26 April, claiming that this was the day Carl XVI Gustaf had been on the throne a day longer than Magnus Eriksson. However, this is based on the date Magnus's successor, Albrecht of Mecklenburg, was acclaimed king, which was not necessarily the same day as Magnus was deposed. In other words, the exact date remains unknown.