Why does Christmas mass usually take place at midnight?

Keeping vigil is an idea familiar to the Scriptures and the Tradition of the Church. Since early times, it has been customary to celebrate Masses late at night or early in the morning for Christmas and Easter as a way of keeping vigilant and waiting for the Lord. In the case of Christmas Eve Mass, it is to wait for Him to be born, which Tradition says occurred at night. In the case of Easter Vigil Mass, it is a way of joining the many generations of the holy ones of the Old Testament in awaiting the Resurrection of the Lord.

Additionally, the Midnight Mass reading (Luke 2:1-14) indicates that the shepherds the angel appeared to were on their nightly vigils watching the sheep.