The Midnight Office or Nocturnes is a liturgical service to be celebrated in the middle of the night. In the Byzantine Rite, it is a monastic office which looks to the coming of the Lord.
The Midnight Office is particularly important on the night between Holy Saturday and Pascha, when it starts (according to tradition) around 11:30 pm. The Canon of the Matins of Holy Saturday is solemnly sung while the Burial Shroud is removed from the tomb and taken in procession to the Holy altar. This service marks the end of the Lenten Triodion and the beginning of Pascha. (In parishes where the Midnight Office is not celebrated, this procession takes place during the Paschal matins.)
Since Matins begins with a nocturnal vigil, it is possible that the Midnight Office was a later addition to fill the “space” between Compline and Matins.
The Midnight Office makes an excellent prayer service for those of the faithful who wish to “watch and Pray” during the night.
Adapted from: www.metropolitancantorinstitute.org