On the traditonal Roman calender, July 12 is the feast day of St. Veronica.
On the road to Calvary, tradition says St. Veronica experienced the pain and agony Our Lord was undergoing in much the same way His Blessed Mother Mary felt His suffering. Veronica saw Jesus carrying His cross to Golgotha and was moved with pity. Offering Him her veil, Jesus wiped His face and gave it back to her. She then noticed the miraculous imprint of the Holy Face on the cloth.
Though not mentioned in Sacred Scripture, this event is recorded in the visions of Bd. Anne Catherine Emmerich. According to the The Dolorous Passion of Our Lord Jesus Christ, "Jesus took the veil in His hand, wiped His bleeding face, and returned it with thanks. Veronica kissed it, and placed it under her cloak."
According to Professor Antonio Paolucci, former director of the Vatican Museums, Veronica's Veil was held as a precious treasure of the popes, kept in St. Peter's Basilica from the year 706 through 1527.
A prayer written in 1208 by Pope Innocent III captures the essence of the devotion:
God, who didst wish to leave as a memorial of Thee to us, who are marked with the light of Thy countenance, an image impressed upon a cloth at the urging of Veronica: Through Thy passion and Cross grant us, we beseech Thee, that we may now upon the earth be so able to venerate and adore it through a glass darkly, that in safety may we see Thee face to face when Thou comest to judge.
Since the early 1600s, this "true icon," or icona vera, a derivation of the name "Veronica," has supposedly been seen by few. Multiple versions and images of the veil appeared over the centuries, with at least six images claimed to be the true veil.
The veil kept in St. Peter's Basilica, removed in the Middle Ages, has since been returned, where it's kept in the chapel behind the balcony of the southwest pier upholding the dome. Each year, the Holy Face is brought out for display on the Fifth Sunday of Lent, with a blessing occurring after Vespers, followed by a procession inside the basilica.