Princess Astrid, Mrs. Ferner is the older sister of a king who ascended to the throne on which day ?
Princess Astrid, Mrs. Ferner:  She is the older sister of King Harald V of Norway and younger sister of the late Princess Ragnhild. Harald V of Norway: Harald V (] ; born 21 February 1937) is the King of Norway, having ascended the throne following the death of his father on 17 January 1991.
on 17 January 1991