Luke 2:21 says that Jesus Christ was formally named and circumcised on the eighth day of his life. January 1 was once celebrated as the Feast of the Circumcision of Christ (these days Roman Catholics observe January 1 as the Solemnity of Mary instead). The apocryphal Arabic Gospel of the Infancy of the Savior goes into much more detail about what happened, saying that when Jesus was a baby,
"And the time of circumcision, that is, the eighth day, being at hand, the child was to be circumcised according to the law. Wherefore they circumcised Him in the cave. And the old Hebrew woman took the piece of skin; but some say that she took the navel-string, and laid it past in a jar of old oil of nard. And she had a son, a dealer in unguents, and she gave it to him, saying: See that thou do not sell this jar of unguent of nard, even although three hundred denarii should be offered thee for it. And this is that jar which Mary the sinner bought and poured upon the head and feet of our Lord Jesus Christ, which thereafter she wiped with the hair of her head.
On the other hand, Saint Birgitta of Sweden
had a vision where the Virgin Mary
told her that she had worn her son's foreskin as jewelry
throughout her life, and before herself ascending bodily to heaven, passed it on to St. John the Evangelist
Whichever story they chose to believe, Christians of the Middle Ages accepted that Jesus's foreskin had been saved by people who knew it was from a holy person. By the time Christianity had spread across Europe, there were multiple places which claimed to have the foreskin of Jesus available for religious pilgrims to visit, view, and hope that it would work miracles.
The first known mention of a Holy Prepuce as a relic is from about the year 800, when Charlemagne gave it to Pope Leo II after Leo had crowned him Holy Roman Emperor. Some stories say Charlemagne claimed angels or the Christ Child had given the relic to him when he visited Jerusalem; others said an unknown man had delivered it to him in the Holy City. It may actually have been a gift from Empress Irene of Byzantium (an odd way to sweet-talk a fellow political leader!), since previous rulers of Constantinople/Byzantium had collected a lot of relics. This one stayed in Rome at the Lateran Chapel until Charles V's armies sacked the city in 1527, when it disappeared, along with a lot of other items of either religious or earthly value.
Thirty years later, so the story goes, outside the small village of Calcata north of Rome, donkeys and goats started to stop in the middle of the main path into town and bow their heads outside a cave used for storage. (The cave had been fitted with doors to keep the barrels and hay bales inside from being stolen, and had at one point been used as a jail.) After the animals had apparently been bowing long enough to cause a lot of comment, the local priest unbolted the doors and looked around the cave. In a corner, partly hidden by hay and manure, he found a silver box "half the size of a palm long and three fingers high." Inside the box were three small sacks, each tied with a red ribbon.
The priest took the box and its contents to the local nobles. Together, they read the writing on the ribbons: one sack was labeled as having the big toe of Saint Valentine, one the jawbone of Saint Martha, sister of Mary, and the third label was too worn to be completely made out. The woman who tried to open the sack said that her hand went numb when she pulled on the ribbons. The purest person available, the seven-year-old daughter of the family, was summoned and was able to open the bag without numbness, and revealed an item that resembled "a red chickpea," which apparently started to exude a sweet-smelling mist that enveloped the village for a couple of days. The clergy and nobility knew what relics had disappeared in Rome and figured that this was the Lateran Foreskin. The "N" and "S" that were all that was readable on the worn label were guessed to be the remnants of "Nostro Signore Gesu," or "Our Lord Jesus" in Italian. It was said that the box with these relics had been left in the cave when a German mercenary had been imprisoned there after he wandered into the village fleeing Rome with the loot he'd stolen.
The Lateran Foreskin stayed in Calcata, with the local chapel expanded into a church in its honor, and was still there until it was stolen or disappeared in the 1980s. Other supposed Holy Foreskins were not so lucky. The other best-known one was at the French monastery of Charroux (which, along with the nearest village, was named after it -- the name means "red skin"). Others were in Coulombs, France (that one made a trip to England because it was supposed to ease childbirth and Henry V's wife Catherine was pregnant) and Conques, Spain. One in Antwerp, now part of Belgium, was unusual in that it was not supposed to have been inherited from Charlemagne, but brought from Jerusalem by Godfrey of Bouillon during the First Crusade.
Agnes Blannbekin, a 14-century Austrian who lived as a beguine, said to a monk who was writing down her visions that while taking Communion on the Feast of the Circumcision she started to wonder where the real Holy Foreskin was and that suddenly her mouth was filled with sweetness and "a little piece of skin like the skin of an egg," which she swallowed, and then this repeated itself numerous times. The writings of her visions were often censored due to this part being considered downright pornographic.
Pope Innocent III declined to say which of the many claimants was the "real" foreskin, saying only God could know. Saint Birgitta of Sweden and Saint Catherine of Siena (who said she was the spiritual bride of Christ and was given the foreskin to wear around her ring finger) seemed to favor the Lateran foreskin, but Pope Clement VII gave an indulgence to those who came to venerate the Charroux foreskin (though he would later be labeled an antipope at the end of the schism between the popes in Avignon and those in Rome). Apparently there were enough claimants around that (according to Marc Shell's Art and Money) "a properly trained physician chosen by the local priest would taste the shriveled leather to determine whether it was wholly or partly human skin."
Skepticism about any of the Holy Foreskins being real had been growing. Some theologians argued that Jesus's body was restored, made perfect, before he ascended into heaven, and therefore his foreskin would have been restored to his body. Greek theologian Leo Allatius thought the foreskin ascended into heaven, but not with the rest of Jesus; he thought it flew out and expanded to become the rings of Saturn. The Protestant reformation generally made relics seem like superstition, if not actual idolatry; John Calvin wrote a tract against them and snarkily wondered how large Jesus' penis must have been if the foreskin could produce so many pieces.
By 1900, most of the supposed relics had been lost or destroyed, and the Roman Catholic Church made it a crime publishable by excommunication to talk or write about the Holy Foreskin. This was justified on the basis that stories about it encouraged "an irreverent curiosity." The one in Calcata, however, was paraded through the village in its box on the Feast of the Circumcision every year until it disappeared -- the village priest said it had been stolen, though he never filed a police report. Theories also exist that the Vatican took it from Calcata because the relic was an embarrassment to the modern church. David Farley's An Irreverent Curiosity speculates that the relic is just as likely to be hidden in the Calcata's church's crypts, but he came to no firm conclusion about where the last known Holy Foreskin might be now.
"The Arabic Gospel of The Infancy of The Saviour" http://wesley.nnu.edu/Biblical_Studies/noncanon/gospels/infarab.htm
Calvin, John. "Treatise on Relics." http://www.godrules.net/library/calvin/176calvin4.htm
Cline, Austin. "Holy Foreskin! Whatever Happened to Jesus’ Foreskin?" http://atheism.about.com/od/aboutjesus/a/holyforeskin.htm
Farley, David. An Irreverent Curiosity: In Search of the Church's Strangest Relic in Italy's Oddest Town. New York: Gotham Books, 2009.
Manseau, Peter. Rag and Bone: A Journey Among the World's Holy Dead. New York: Henry Holt and Company, 2009.
Shell, Marc. Art and Money. University of Chicago Press, 1995. http://books.google.com