In 325 CE at the Nicean Council, Emperor Constantine ordered another revision of the calendar. Finally, around 523 CE, the Vatican asked a monk by the name of Dionysius Exiguus to devise a way to implement Constantine's order for general use. At that time it was customary to count years since the reign of Roman Emperor Diocletian...so, in his calculations Dionysius chose to number the years since the birth of Christ instead of honoring Diocletian. Dionysius (wrongly) fixed Jesus' birth with respect to Diocletian's reign in such a manner that it falls on December 25, 753 AUC (ab urbe condita, i.e. since the founding of Rome), thus making year 1 of the current era start on January 1, 754 AUC. How Dionysius established the year of Christ's birth is not known...the bible says Jesus was born under the reign of King Herod the Great, who died in 750 AUC, which means that Jesus could have been born no later than that year. Dionysius' calculations were disputed at a very early stage. When people started dating years before 754 AUC (using the term "Before Christ"), they let the year 1 BC immediately precede the year 1 with no intervening year zero.

Then, in 567 CE the Council of Tours abolished January 1st in favor of March as the start of a new year, varying the actual day to coincide with the Vernal Equinox. After this, 1,000 years passed before another change in the calendar occurred. In 1582 Pope Gregory XIII undertook the revision now in general use and the first day of the new year was moved back to January 1st.

In the first centuries of the current era, the Romans continued celebrating the new year and the early Christian Church condemned the festivities as paganism. But as Christianity became more widespread, the early Church began having its own religious observances concurrently with many of the pagan celebrations...New Year's Day was no different. The tradition of using a baby to signify the new year was begun in Greece around 600 BCE...it was their tradition to celebrate the new year by parading a baby in a basket that represented the annual rebirth of their god of fertility. Early Egyptians also used a baby as a symbol of rebirth. Although the early Christians denounced the practice as pagan, the popularity of the baby as a symbol of rebirth eventually forced the Catholic Church to reevaluate its position and they finally allowed its members to celebrate the new year with a baby which was to symbolize the circumcision of the baby Jesus.

In 1970, the Vatican uncharacteristically moved forward again: Pope Paul VI created a special feast day that simultaneously celebrates the Maternity of Mary and World Peace. Since then, this immensely gentler motive has replaced circumcision as the Church's cause for rejoicing on January 1st.