Lazarus Saturday is the day before Palm Sunday (the feast of Christ’s triumphal entry into Jerusalem). This Saturday and Sunday are the connection between Great Lent and Holy Week. On the eve of the celebration of the Resurrection of Lazarus, the forty days of Great Lent are formally brought to an end at Vespers. These two days are the unique and paradoxical days before the Lord’s Passion.
Celebration of the Feast: Lazarus Saturday is a paschal celebration. The liturgy of Lazarus Saturday glorifies Christ as the Resurrection and the Life who, by raising Lazarus, has confirmed the universal resurrection of mankind, even before his own suffering and death. This liturgy is the only time in the Church year that the resurrectional service of Sunday is celebrated on another day.
Lazarus Saturday was once among the few great baptismal days in the ecclesiastical year. At the Divine Liturgy of Lazarus Saturday the baptismal verse from Galatians: As many as have been baptized into Christ have put on Christ (Galatians 3:27) replaces the Thrice-Holy Hymn.
From: Orthodox Wiki