40th jubilee on the throne is an extremely rare event in Danish history. Throughout the kingdom’s more than 1000-year history, there were only two monarchs before Queen Margrethe II who one can, without reservation, say occupied the throne for 40 years: Christian IV in 1628 (or 1636 if one counts from the coronation) and Christian IX in 1903.
Admittedly, one can also point to Christian VII and his successor, Frederik VI, though not both at once. Their 40 years on the throne depends on a distinction between real and formal reigns. That distinction is related to Christian VII’s insanity, which made him unable to rule. That was why he, in 1784, after reigning for 18 years, faced a coup that deprived him of the real power of government, which was assumed by his son, Crown Prince Frederik. He subsequently governed as the prince regent in the king’s name until his father died in 1808, after which Frederik continued to rule as King Frederik VI until his death in 1839. If asked about formal governance, one can thus say that Christian VII reached his 40th jubilee in 1806; if asked about real governance, one can say that Frederik VI had ruled for 40 years in 1824. Regardless which criterion one chooses, there was only one of them who ruled for 40 years.
Of the 52 Danish monarchs since Gorm The Old, Queen Margrethe is thus only the fourth to reach a 40th jubilee on the throne.