Year of three popes

A year of three popes is a year when the College of Cardinals of the Catholic Church is required to elect two new popes within the same calendar year, in particular the last instance in 1978. Such a year has historically occurred when a newly elected pope dies very early into his papacy. This results in the Catholic Church being led by three different popes during the same calendar year. In one instance, in 1276, there was a year of four popes.

Instances

There have been twelve instances in which exactly three popes have held office in a given calendar year.

827: Eugene II — Valentine — Gregory IV
896: Formosus — Boniface VI — Stephen VI
897: Stephen VI — Romanus — Theodore II
964: Leo VIII — Benedict V — John XIII
1003: Sylvester II — John XVII — John XVIII
1045: Sylvester III — Benedict IX (second reign) — Gregory VI
1187: Urban III — Gregory VIII — Clement III
1503: Alexander VI — Pius III — Julius II
1555: Julius III — Marcellus II — Paul IV
1590: Sixtus V — Urban VII — Gregory XIV
1605: Clement VIII — Leo XI — Paul V
1978: Paul VI — Pope [John Paul I|John Paul I] — John Paul II

There was also a year in which the Roman Catholic Church was led by four popes, called the Year of Four Popes:

1276: Gregory X — Innocent V — Adrian V — John XXI