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 from Matins of Holy Saturday is solemnly sung while the Burial Shroud is removed from the tomb and taken in procession to the Holy Table. This service marks the end of the time of the Lenten Triodion, and the beginning of Pascha. (In parishes where the Midnight Office is not celebrated, this procession takes place during Paschal Matins.)
Since Matins also begins with a nocturnal vigil (the fixed psalmody before the readings), it is possible that the Midnight Office is a later addition to the Byzantine cycle of prayer, to fill the "space" between Compline and Matins. The Midnight Office is omitted if there is an all-night vigil.
The Midnight Office makes an excellent night time prayer service for those of the faithful who wish to "watch and pray" during the night.