The Roman Catholic New Year's Eve service at St. Peter's Basilica in Vatican City is associated with the Te Deum.
Te Deum is a Christian hymn of praise whose Latin title means “You, God.” In the Catholic Church, a Te Deum is traditionally sung at the end of the year as an act of thanksgiving. The Pope has commonly presided over a New Year's Eve service at St. Peter's Basilica or issued a related liturgical observance, with the hymn marking gratitude for the year that has passed.
The service is often connected with First Vespers of the Solemnity of Mary, the Holy Mother of God, which is celebrated on January 1. It may also include the exposition or veneration of the Blessed Sacrament and a papal homily, depending on the year's liturgical program.
Te Deum is a hymn, not the name of the entire calendar celebration. It is also used on other important occasions, including thanksgiving services and national ceremonies. The title can therefore appear in contexts beyond New Year's Eve, but it is the canonical answer for this Vatican year-end tradition.