A watchnight service (also called Watchnight Mass) is a late-night Christian church service. In many different Christian traditions, such as those of Moravians, Methodists, Catholics, Lutherans, Anglicans, Baptists, Adventists and Reformed Christians, watchnight services are held late on New Year's Eve, which is the seventh day of Christmastide. This provides the opportunity for Christians to review the year that has passed and make confession, and then prepare for the year ahead by praying and resolving. The services often include singing, praying, exhorting, preaching, and Holy Communion.

Watchnight services can take the form of Watchnight Covenant Renewal Services, Watchnight Vespers services, Watchnight Vigil services, or Watchnight Masses. As Watchnight services bring in the New Year by glorifying God, they are seen by many Christians as being preferable to "drunken revelry" in popular cultural celebrations that are commonplace in some localities.

In addition to Christian denominational traditions, the ethnic traditions of Koreans and African Americans have a strong tradition of New Year's Eve watchnight services.
The Bible documents that at the time of the Exodus from Egypt, God ordered Moses to "set up the tabernacle of the tent of the congregation" on "the first day of the first month" (Exodus 40:1).

In Christianity, since the time of the early Church, Christians have held vigils (watchnights) before the celebration of feast days, a practice "inspired by Jesus's example of praying all night before important decisions." At that time, non-Christians of the Greco-Roman world observed the arrival of the New Year with "revelling" and Christians distinguished themselves by instead praying and fasting.

Throughout history, Christian denominations including the Catholic Church, Lutheran Church and Anglican Church have variously observed the eighth day of Christmastide—New Year's Day—as the Feast of the Circumcision of Christ, Feast of the Holy Name and Solemnity of Mary, Mother of God, with the evening before having the Vigil Mass (Watchnight Mass) for the feast.

The Moravian Church came to hold a lovefeast on New Year's Eve, followed by a watchnight service in the evening. These watchnight services last three hours and have been held since they became popular in the Czech Republic in 1733.

After attending a Moravian watchnight service on New Year's Eve in 1738, John Wesley, the father of the Methodist Churches, recorded that "as we were continuing instant in prayer, the power of God came mightily upon us, insomuch that many cried for exceeding joy, and many fell to the ground." The Methodist Churches, strongly influenced by the Moravian Church and Radical Pietism in general, herald the practice of the Watchnight Service, with John Wesley having emphasized that it was "customary with the ancient Christians to spend whole nights in prayer".

The practice of holding watchnight services on New Year's Eve became ubiquitous throughout Christendom, with many Christian denominations now offering them.
31 December