In Slavic practice, on the eve of the Nativity of the Lord, the liturgical services consist of the Royal Hours with the Typika, Vespers, and the Divine Liturgy of Saint Basil the Great. At Vigil in the evening, Great Compline is followed by Matins.

Commemorating the Nativity in the flesh of our Lord Jesus Christ, was established by the Church. Its origin goes back to the time of the Apostles. In the Apostolic Constitutions (Section 3, 13) it says, “Brethren, observe the feastdays; and first of all the Birth of Christ, which you are to celebrate on the twenty-fifth day of the ninth month.”

Read more at: https://www.oca.org/saints/lives/2021/12/24/103630-eve-of-the-nativity-of-our-lord