Friedrich Ferdinand, Duke of Schleswig-Holstein
Friedrich Ferdinand of Schleswig- Holstein- Sonderburg- Glücksburg( 12 October 1855 – 21 January 1934) was the fourth Duke of Schleswig- Holstein- Sonderburg- Glücksburg and became Duke of Schleswig- Holstein in 1931.
Wilhelm Friedrich, Duke of Schleswig-Holstein
Friedrich, Duke of Schleswig- Holstein- Sonderburg- Glücksburg, from 1934 Duke of Schleswig- Holstein( 23 August 1891 – 10 February 1965), was the Duke of Schleswig- Holstein and Head of the House of Oldenburg from 21 January 1934 until his death on 10 February 1965.
Joachim Ernest, Duke of Schleswig-Holstein-Sonderburg-Plön
Joachim Ernest of Schleswig-Holstein- Sonderburg- Plön( 29 August 1595 – October 5, 1671), also" Joachim Ernest of Schleswig- Holstein- Plön", was the first Duke of Schleswig- Holstein- Sonderburg- Plön, which emerged from a division of the Duchy of Schleswig- Holstein- Sonderburg.
Princess Alexandra Victoria of Schleswig-Holstein-Sonderburg-Glücksburg
Princess Alexandra Victoria of Schleswig-Holstein- Sonderburg- Glücksburg( 21 April 1887 – 15 April 1957) was a daughter of Friedrich Ferdinand, Duke of Schleswig- Holstein and his wife Princess Karoline Mathilde of Schleswig- Holstein- Sonderburg- Augustenburg.
Ernst Gunther, Duke of Schleswig-Holstein
Ernst Gunther, Duke of Schleswig-Holstein (August 11, 1863 – February 22, 1921) was a son of Frederick VIII, Duke of Schleswig-Holstein and Princess Adelheid of Hohenlohe-Langenburg.
He inherited his father's title as titular duke of Schleswig-Holstein.
On August 2, 1898 in Coburg, he married Princess Dorothea of Saxe-Coburg and Gotha, daughter of Prince Philipp of Saxe-Coburg and Gotha and Princess Louise of Belgium.
The grandfather of the bride, King Leopold II, did not attend the wedding but sent the Grand Cordon of the Order of Leopold as wedding gift.
The couple had no children.
However, on 11 November 1920, Ernst Gunther adopted Prince Johann Georg and his sister Princess Marie Luise, children of Prince Albrecht of Schleswig-Holstein-Sonderburg-Glücksburg.
Joachim Frederick, Duke of Schleswig-Holstein-Sonderburg-Plön
Joachim Frederick of Schleswig-Holstein- Sonderburg- Plön( 9 May 1668, Magdeburg – 25 January 1722, Plön), also known as Joachim Frederick of Schleswig- Holstein- Plön, was the third Duke of Schleswig- Holstein- Plön, a dukedom created by the division of the Duchy of Schleswig- Holstein- Sonderburg.
Princess Dorothea of Saxe-Coburg and Gotha
Princess Dorothea Maria Henriette Auguste Louise of Saxe-Coburg and Gotha (30 April 1881 – 21 January 1967) was a princess of Saxe-Coburg and Gotha  by birth and the duchess of Schleswig-Holstein  through her marriage to Ernst Gunther, duke of Schleswig-Holstein.
Dorothea was born in Vienna, Austria, the second child and only daughter of Prince Philipp of Saxe-Coburg and Gotha and Princess Louise of Belgium.
Albert, Duke of Schleswig-Holstein
Albert, Duke of Schleswig- Holstein( Albert John Charles Frederick Alfred George; 26 February 1869 – 27 April 1931), was a grandson of Queen Victoria.
He was the second son of Victoria's daughter, Princess Helena, by her husband Prince Christian of Schleswig- Holstein.
He was the head of the House of Oldenburg and also the House of Schleswig- Holstein- Sonderburg- Augustenburg and the Duke of Schleswig- Holstein between 1921 and 1931.
Princess Adelheid of Hohenlohe-Langenburg
Princess Adelheid of Hohenlohe-Langenburg (20 July 1835 – 25 January 1900) was Duchess of Schleswig-Holstein, a niece of Queen Victoria of the United Kingdom, a cousin of King Edward VII, and the mother-in-law of Emperor Wilhelm II of Germany.
She is the direct most recent common matrilineal ancestress (through women only) of Carl XVI Gustaf of Sweden and Felipe VI of Spain.
Friedrich, Duke of Schleswig-Holstein-Sonderburg-Glücksburg
Friedrich of Schleswig- Holstein- Sonderburg- Glücksburg( 23 October 1814 in Schleswig, Duchy of Schleswig – 27 November 1885 in Luisenlund, Schleswig- Holstein, Prussia) was the third Duke of Schleswig- Holstein- Sonderburg- Glücksburg.
Friedrich was the second- eldest son of Friedrich Wilhelm, Duke of Schleswig- Holstein- Sonderburg- Glücksburg and Princess Louise Caroline of Hesse- Kassel and an elder brother of Christian IX of Denmark.
Friedrich inherited the title of Duke of Schleswig- Holstein- Sonderburg- Glücksburg upon his childless brother Karl's death on 14 October 1878.

Question: What is the date of death of Ernst Gunther, Duke Of Schleswig-Holstein's mother?
Answer: 25 January 1900