Double (1955 Calendar): May 19
There are actually two canonized popes with the name of Celestine - St. Celestine I who reigned from 422 to 432 and St. Celestine V who reigned in 1294. Today's feast of St. Peter Celestine refers to Pope St. Celestine V.
St. Peter Celestine was Pope for five months from 5 July to 13 December 1294, when he resigned. He was also a monk and hermit who founded the order of the Celestines. It is noted that he is one of only a few popes to have ever resigned from office.
On December 13, 1294, clothed in full pontifical vesture, he read before the Cardinals this act of his great renunciation:
“Inspired by many legitimate reasons, desiring a more humble state and a more perfect life, fearing to compromise my conscience and seeing my weakness and incapacity, considering the malice of men and yearning for the rest and spiritual consolation I enjoyed before I was raised to this position, I, Celestine V, Pope, do hereby freely and voluntarily renounce the Sovereign Pontificate and abandon the dignity and position to which I was raised.”In 1313, St. Peter Celestine was canonized and since then no pope has ever again taken the name of Celestine.
To mark the 800th anniversary of Celestine's birth, Pope Benedict XVI proclaimed the Celestine year from 28 August 2009 through 29 August 2010.
His entry in the Martyrologium Romanum for 19 May reads as follows:
At Castrum Fumorense near Alatri in Lazio, the birth of Saint Peter Celestine, who, when leading the life of a hermit in Abruzzo, being famous for his sanctity and miracles, was elected Roman Pontiff as an octogenarian, assumed the name Celestine V, but abandoned his office that same year and preferred to return to solitude.Collect:
O God, who hast raised blessed Peter Celestine to the supreme Pontificate, and also taught him to prefer the virtue of humility, mercifully grant that following his example, we may despise the things of this world, and so merit to attain those rewards which Thou hast promised to the humble of heart.
Prayer Source: 1962 Roman Catholic Daily Missal