Information is from the Canon Regulars of St. John Cantius:
In the year 320, Constantine placed here the relics of the Holy Cross, which his mother, St. Helen, had brought to Rome from the Holy Land. Also, there is soil brought from Calvary, placed under the floor of the Chapel of the Holy Cross. Today, in the Church of Calvary at Rome—that is of the Cross—our hope, the Church, sends a ray of light upon our souls to stir us up to persevere in the struggle against the world, the flesh and the devil, until the great feast of Easter is reached.
"Rejoice, rejoice with joy," we are told in the Introit, for having died to sin with our Lord during Lent, we are shortly to rise with him by the Paschal Confession and Communion.
Our whole life is a texture of sorrows and joys. Good Fridays and Easters accompany us on our journey to the land of perennial Easter. But as there is no Good Friday without the assurance that "by the wood of the Cross joy has come into the whole world," so in the soul of a true Christian there is no sorrow without the joy that will come from living faith, strong hope and sincere love. It is a joy ever sustained and increased by that wonderful Bread, which Christ's loving hand multiplies for us in this desert of life.
By the wood of this Cross joy has come into the world and into your heart, also. Lætare Sunday, Jerusalem! Endure the thorns of life courageously. Supernaturalize them.
(Note: On this day, it was the custom to solemnly bless the "golden rose," which was then presented by the Holy Father to a Catholic, who was zealous and outstanding in the Faith.)