The History of the Popes (Vol. XX) : From the Close of the Middle Ages examines the pontificate of Pope Gregory XIII between 1572 and 1585. This volume focuses on various significant aspects of the Catholic Church's history in the late 16th century, including the continued implementation of the Council of Trent's decrees and rigorous internal Church reform; the history behind Pope Gregory XIII’s major initiative to revise the calendar still in use today; and the Holy See’s navigation of European political turmoil including dynamics with England and France, as well as missionary efforts in the East. This comprehensive work spans a total of 40 volumes chronicling the history of 56 popes.