Christ, the I AM said "Your father Abraham rejoiced to see my day: and he saw it, and was glad." Abraham saw (by faith) the eternal God in His humiliation. He saw the day of Christ's perfect obedience to His own holy law, thereby producing a fit righteousness that would eternally robe every child of promise. He saw the day of substitution where Christ, the sacrificial Lamb, would die under the hand of holy Justice because of the elect's imputed guilt to His account. Abraham saw the day of Christ's vindication when the tomb could no longer hold Him captive, Eternal Justice having been satisfied; the Father raised Christ from the dead, justified, and with Him all for whom Christ died.
Eternity Himself revealed His day to some sinners. Abraham saw it and was glad.