Resignation of the Past Popes

The best known example involved Pope Celestine V in 1294. After only five months as the Bishop of Rome, he issued a solemn decree declaring it possible that a pope can resign and then promptly did so. He then lived the rest of his days as a hermit and was later canonised.
His papal decree ended any debate over whether a pope can justify standing down during his lifetime.
Before Celestine, the only other two pontiffs to resign were the current Pope's namesake Benedict IX in 1045 and his successor Gregory VI the year after.
Gregory had schemed to get rid of Benedict, who had been beset by a succession of financial scandals. After persuading the disgraced incumbent to resign in 1045, and then becoming his successor as Gregory VI, he also resigned the following year after he was accused of benefiting financially from Benedict's demise.
The last time a pope resigned was Pope Gregory XII in 1415. He stood down to end the "Western Schism", which threatened to shatter Roman Catholicism. Two rival claimants had declared themselves pope in Avignon and Pisa and, with the help of the wily Italian politician Malatesta, Gregory's resignation helped unite the church at the Council of Constance in 1415.