First liturgy of Christmastide that is celebrated on the night of Christmas Eve
In many Western Christian traditions, Midnight Mass is the first liturgy of Christmastide that is celebrated on the night of Christmas Eve, traditionally beginning at midnight when Christmas Eve gives way to Christmas Day. This popular Christmas custom is a jubilant celebration of the mass or service of worship in honour of the Nativity of Jesus; even many of those Christian denominations that do not regularly employ the word mass uniquely use the term "Midnight Mass" for their Christmas Eve liturgy as it includes the celebration of Holy Communion. History.
https://en.wikipedia.org/wiki/Midnight_Mass