While St. Francis of Assisi, in September 1224, was rapt in meditation on Mount Alvernia, Almighty God as a mark of His special favor deigned to impress on his hands, feet and side the likeness of the sacred wounds of Christ. Until the day of the saint’s death blood flowed from these wounds at intervals. Pope Sixtus V ordered this feast to be observed by the whole Church. Clement VIII suppressed it, because the Church only dedicates special solemnities to the mysteries of our redemption; particular favors granted to the saints by God directly concern their individual sanctification, and are commemorated when the life of each saint is read in the Breviary on the day of his feast. The feast was restored, however, by Paul V and raised to a rank of a double by Clement XIV. Its observance is a privilege overstepping the ordinary rules of the Liturgy, extended in honor of St. Francis and not only rare, but also unique. Christ was pleased to imprint upon his new apostle, the herald of the Great King, this stigmata as His final seal, changing him to His own image and likeness and uniting him to Himself on the rood of the Cross.
(Source: The New Roman Missal, Rev. F. X. Lasance)