premise: Henry II (18 April 1503 – 25 May 1555), nicknamed "Sangüesino" because he was born at Sangüesa, was the King of Navarre from 1517, although his kingdom had been reduced to a small territory north of the Pyrenees by the Spanish conquest of 1512. Henry succeeded his mother, Queen Catherine, upon her death. His father was her husband and co-ruler, King John III, who died in 1516.
hypothesis: King John III died on 18 April 1516.
reason: The date and month of his death are never mentioned, so we don't know for sure. It's difficult because April 18 is mentioned elsewhere in the text.