Pope Pius IV reopens the Council of Trent for its third and final session.

Pope Pius IV, born Giovanni Angelo Medici, was head of the Catholic Church and ruler of the Papal States from 25 December 1559 to his death, in December 1565. Born in Milan, his family considered itself a branch of the House of Medici and used the same coat of arms. Although modern historians have found no proof of this connection, the Medici of Florence recognised the claims of the Medici of Milan in the early 16th century.

The Council of Trent, held between 1545 and 1563 in Trent, now in northern Italy, was the 19th ecumenical council of the Catholic Church. Prompted by the Protestant Reformation at the time, it has been described as the "most impressive embodiment of the ideals of the Counter-Reformation." It was the last time a Catholic ecumenical council was organized outside the city of Rome, & the second time to be convened in the territory of the Holy Roman Empire.