"It may also be chanted or recited before the beginning of Christmas Mass during the Night. It may not replace any part of the Mass." (emphasis mine)
50. When the Entrance Chant is concluded, the Priest stands at the chair and, together with the whole gathering, signs himself with the Sign of the Cross. Then by means of the Greeting he signifies the presence of the Lord
to the assembled community. By this greeting and the people’s response, the mystery of the Church gathered together is made manifest.
After the greeting of the people, the Priest, or the Deacon, or a lay minister MAY very briefly introduce the faithful to the Mass of the day.
THE NATIVITY OF OUR LORD JESUS CHRIST
from the Roman Martyrology
[...]
This text, The Nativity of our Lord Jesus Christ, may be chanted or recited, most appropriately
on December 24, during the celebration of the Liturgy of the Hours. It may also be chanted or
recited before the beginning of Christmas Mass during the Night. It may not replace any part of
the Mass.
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