Prosabbaton (G4315) is the day before the Sabbath — what we call Friday, the Preparation Day. It appears in Mark 15:42 in the account of Jesus' burial: Joseph of Arimathea took down the body because it was the prosabbaton, and the Sabbath was about to begin at sunset.
The prosabbaton — Preparation Day — is the day the Lamb was slain. The theological resonance is staggering: every Sabbath was preceded by a day of preparation, and the ultimate Sabbath rest of God (Heb 4:9-10) was preceded by the ultimate act of preparation — the cross. The Passover lambs were slain on the preparation day (Jn 19:14); Jesus, the Passover Lamb (1 Cor 5:7), died when they were being killed. The urgency of Joseph of Arimathea becomes a liturgical act: the body must be in the tomb before Sabbath begins, and so the greatest Rest in history — three days in the tomb — begins on the holy seventh day. Creation's rest becomes redemption's rest.