The Seventh Day of Christmas - Feast of Pope St. Sylvester I
Pope St. Sylvester I was born in Rome to Rufinus and Justa in the late 200's.
He was educated by a priest named Charitius or Carinus in literature and theology and was ordained a priest by Pope St. Marcellinus.
He witnessed Constantine's triumph in 312 and upon the death of Pope St. Melchiades was elevated to the papacy in 314. The same year he sent four legates to the Western Council held at Arles. The Donatist schism and the Quartodeciman heresy were condemned at this council and Pope Sylvester approved the canons written at the council for the whole Church.
He was responsible for the building of the original Basilica of St. Peter's and St. John Lateran and may also have been responsible for creating the first martyrology.
In 325 the General Council of Nicea was convened to deal with the Arian Heresy. The Arians professed that Christ was not truly God and this heresy became so prevalent that for a time most of the Christian world accepted the teachings. The Pope was not able to attend the council because of his age but sent three priests, Osius, Vito and Vincentius to act in his stead. Showing the importance of the pope's representatives, these three priests are listed ahead of the Eastern patriarchs in the list of those attending the council. The council condemned Arianism and drew up the Nicene Creed which is recited at every Mass.
Read more about Pope St. Sylvester here