In the month of February, Ancient Rome celebrated the festival of the Lupercalia. It was “the last publicly tolerated remnant of the heathen faith” . According to Ferdinand Gregorovius , a historian who specialized in the medieval history of Rome, the festival was “centred round the sanctuary of the Lupercal or the wolf-averting Abolition Pan”. It was the oldest of all the Roman sanctuaries, “a dark cave at the foot of the Palatine”. The feast “was celebrated every year on February 15, and was followed on the 18th by the Februatio, or purifying of the city from the influence of demons”. The Lupercal was able to survive when all other ancient festivals “had yielded to the influence of Christianity”. So great “being the reverence of the Romans for this, the most ancient of their national customs, that even as Christians they could not renounce it” and “to the horror of the Bishop it was still celebrated” even after nearly five hundred years “passed since Paul preached the gospel in Rome” .
But why was the Bishop so horrified?
Ferdinand Gregorovius described the main feature of the festival in a following way (the bold font is mine):
The Luperci (youths, members of the sacred college) uncovered themselves unabashed before the eyes of the people, and clad only with an apron of the skins of the goats slain in the sacrifice, ran from the Lupercal through the streets, swinging straps of leather, with which they hit the women strokes on the right hand, thereby to bestow the blessing of fruitfulness. 
Let’s turn to ancient sources. We have Plutarch’s record of this feast.
At this time many of the noble youths and of the magistrates run up and down through the city naked, for sport and laughter striking those they meet with shaggy thongs. And many women of rank also purposely get in their way, and like children at school present their hands to be struck, believing that the pregnant will thus be helped to an easy delivery, and the barren to pregnancy. 
Another translation slightly smooths out the overall picture:
On this occasion many of the young nobles and magistrates run through the city without their toga… 
In order to clarify the character of the festival, I’ll refer to Ovid.
You ask, Why then do the Luperci run? and why do they strip themselves and bear their bodies naked, for so it is their wont to run? The god himself loves to scamper, fleet of foot, about the high mountains, and he himself takes suddenly to flight. The god himself is nude and bids his ministers go nude: besides, raiment sorted not well with running. 
Ovid believed the feast took us to ancient times when
there was as yet no use for horses, every man carried his own weight: the sheep went clothed in its own wool. Under the open sky they lived and went about naked, inured to heavy showers and rainy winds. Even to this day the unclad ministers recall the memory of the olden custom and attest what comforts the ancients knew. 
He also tells a “merry tale” “handed down from days of old” which explains why “betrayed by vesture, the god loves not garments which deceive the eye, and bids his worshippers come naked to his rites“. 
In the end, the bishops got what they wanted and put an end to the “horror”. Pope Gelasius, who was appointed in March 492, insisted that the Romans must understand that “they could not at the same time eat at the table of the Lord and at that of demons, nor drink from the chalice of God and that of the devil.” “It is probable that the zeal of the Pope succeeded in inducing the Senate to abolish the Lupercalia.” The Church “transformed the old festival of purification in the Lupercalia into the feast of the Purification of Mary” (cp. ).
If we accept Ovid’s interpretation of the meaning of the feast, the abandonment of clothing will appear as part and parcel of the celebration. In ancient times, nudity often was considered as a means of ritual purification. Even in ancient Christianity, “the process of taking off clothes was an essential moment of the baptismal ceremony, as it was structured during the 4th century” . Maybe these days we are witnessing the rebirth of the ancient attitude towards nudity. I have in mind, of course, the World Naked Bike Ride  and similar events. Who knows? Maybe they’ll become something big: new festivals of purification. The “garments” “deceive the eye”, after all.
 Ferdinand Gregorovius, History of the City of Rome in the Middle Ages, Cambridge University Press, 2010
 Ferdinand Gregorovius – Wikipedia
 Plutarch, The Parallel Lives. The Life of Julius Caesar, Vol. VII of the Loeb Classical Library edition, 1919
 Plutarch, Plutarch’s Lives Volume III, George Bell & Sons, 1892 – The Project Gutenberg
 Ovid, Fasti. Book 2 – Theoi Project – Classical Texts Library
 According to another unverified hypothesis, the festival of Lupercalia was re-branded as “St. Valentine’s Day” (see Drew Miller, Ancient History of Valentine’s Day Reveals a Super Kinky Past
Valentine’s Day – Wikipedia
 World Naked Bike Ride – Wikipedia
 Giovanni Filoramo, Baptismal Nudity as a Means of Ritual Purification in Ancient Christianity. In: Transformations of the Inner Self in Ancient Religions