Wenceslaus II, Duke of Cieszyn
Wenceslaus II of Cieszyn( b. 1488/96- d. 17 November 1524), was a Duke of Cieszyn since 1518 until his death( as co-ruler of his father).
He was the second son of Casimir II, Duke of Cieszyn, by his wife Johanna, daughter of Wiktoryn of Poděbrady, Duke of Opawa.
Leszek, Duke of Masovia
Leszek of Masovia( also" Lestek")( b. ca. 1162- d. 1186) was a Polish prince from the Piast dynasty, the Duke of Masovia from 1173 until his death.
He was the only son of Bolesław IV the Curly, Duke of Masovia and High Prince of Polan, who survived father.
After his father death he inherited Masovia.
At the beginning, Leszek ruled under the guardianship of his uncle Casimir II the Just.
He was a man of poor health.
For a short time he supported his other uncle, Mieszko III the Old, but later decided to reconciled with Kazimierz II, who after Leszek's death inherited his duchy.
Adelaide of Poland
Adelaide of Poland( c. late 1170s/ early 1180s – 8 December 1211), was a Polish princess and member of the Piast dynasty.
She was the daughter of Casimir II the Just, Duke of Sandomierz and High Duke of Poland, by his wife Helena of Znojmo, a Přemyslid princess.
On basis of the inscription of her tombstone at the Dominican Church and Convent of St. James in Sandomierz and two different reports of Jan Długosz, modern historians agreed with the origin and facts from Adelaide's life.
Today existed a dominant view in historiography consistent with her filiation and death date.
According to them, Adelaide was indeed the daughter of Casimir II the Just and died in 1211, but was n't the foundress of the Convent of St. James and only another nun there.
In the 19th century, appeared a theory which states that Adelaide was the daughter of Casimir I of Kuyavia, who entered in the Convent of St. James as a nun and died there in 1291.
This view, accepted by several scholars, has been disputed.
More recent historiography recognized her as the daughter of Casimir II the Just, founder of the Convent, in which was she buried after her death in 1211.
Casimir II of Łęczyca
Casimir II of Łęczyca( pl:" Kazimierz II łęczycki"; c. 1261/62 – 10 June 1294), was a Polish prince member of the House of Piast, Duke of Brześć Kujawski during 1267 – 1288, Duke of Dobrzy ń during 1275 – 1288 and Duke of Łęczyca since 1288 until his death.
He was the fourth son of Casimir I of Kuyavia, but the second born from his third marriage with Euphrosyne, daughter of Casimir I of Opole.
He was probably named after both his father and maternal grandfather.
Siemowit III, Duke of Masovia
Siemowit III of Masovia( his name also rendered Ziemowit; – 1381) was a prince of Masovia and a co-regent( with his brother Casimir I of Warsaw) of the lands of Warsaw, Czersk, Rawa, Gostynin and other parts of Masovia.
Alexandra of Lithuania
Alexandra (died 20 April 1434 in Płock) was the youngest daughter of Algirdas, Grand Duke of Lithuania, and his second wife, Uliana of Tver.
Though Alexandra's exact date of birth is not known, it is thought that she was born in the late 1360s or early 1370s.
In 1387, she married Siemowit IV, Duke of Masovia, and bore him thirteen children.
Casimir II of Belz
Casimir II of Belz
(pl: "
Kazimierz II bełski"; 1401/03 – 15 September 1442), was a Polish prince member of the House of Piast from the Masovian branch.
He was a Duke of Płock, Rawa Mazowiecka, Gostynin, Sochaczew, Belz, Płońsk, Zawkrze and Wizna during 1426–1434 jointly with his brothers, and after the division of the paternal inheritance between him and his brothers in 1434, sole ruler over Belz.
He was the third son of Siemowit IV, Duke of Masovia and Alexandra of Lithuania, daughter of Algirdas.
Anna of Masovia
Anna of Masovia( c. 1498 – after 26 January 1557 in Jaroslaw) was Princess of Mazovia and a member of the House of Piast.
She was the last representative of the line of Masovian- Piast.
She was the second daughter of Konrad III of Masovia and Anna Radziwiłł.
She was a sister of Dukes Janusz III of Masovia and Stanislaus of Masovia, and Sophia, who married Stephen VII Báthory.
Kunigunde of Bohemia
Kunigunde of Bohemia( January 1265 – 27 November 1321) was the eldest daughter of Ottokar II of Bohemia and his second wife, Kunigunda of Slavonia.
She was a member of the Přemyslid dynasty.
She was Princess of Masovia by her marriage to Boleslaus II of Masovia and later became abbess of the St. George's Convent at Prague Castle.
Sophia of Masovia
Sophia of Masovia( 1497/1498- before 11 March 1543) was a Princess of Masovia, daughter of Konrad III Rudy, Duke of Masovia and his wife Anna Radziwiłł.
She was a member of the House of Piast.
On 17 September 1520 in Warsaw, Sophia was married by proxy to Stephen VII Báthory.
On 17 January 1521, Sophia and her entourage went to Hungary, where in February there was a formal marriage.
The couple had three children:
Anna, Stephen and Klara; however, other sources claim that Klara was the couple's only child.
Klara was engaged to Carl, Duke of Münsterberg, but died in 1535, before the marriage took place.
After the death of Stephen in 1530, Sophia was married to Croatian magnate Louis Pekry.
Sophia's second marriage was childless.
She then died sometime before 11 March 1543.
Sophia had three siblings: Stanislaus I of Masovia, Janusz III of Masovia and Anna of Masovia, both her brothers died childless and so Anna and Sophia were the last Masovian Piasts.
Soon after Janusz's death, the Duchy of Masovia was annexed into the kingdom of Poland, despite resistance from some of the Masovian nobility.
They tried to retain their independence and argued that the Duchy should be inherited by the female relatives( such as Anna or Sophia).
The Polish king refused to recognize their demands.
Anna was made titular Duchess of Masovia.

Question: What is the date of death of Casimir Ii Of Belz's mother?
Answer: 20 April 1434