From the book: The Life of St. Francis, by Thomas of Celano, in the year 1215, December 13
The Body of our Holy Father St. Francis was found in a church which was built at Assisi in honor of Saint Francis. It was a double church, and the body of the saint was buried deep under the lower church. In the course of time the exact location of the tomb was forgotten; and with the permission of the Holy See, excavations were made in 1818 for the purpose of finding the relics. After 52 nights of hard work, the stone coffin containing the bones and ashes of Saint Francis was found. A third underground church was then hewn out of the solid rock upon which the church had been built; and there the relics of Saint Francis are venerated today. Pope Leo XII instituted a special feast to commemorate the finding of the body of Saint Francis. It is observed on December 12, except in the Americas where it is kept on December 13.