12 The Pentecost, too, we observe, that is, the fiftieth day from the passion and resurrection of the Lord, for on that day He sent to us the Holy Paraclete whom He had promised; as was prefigured in the Jewish passover, for on the fiftieth day after the slaying of the lamb, Moses on the mount received the law written with the finger of God. If you read the Gospel, you will see that the Spirit is there called the finger of God. Remarkable events which happened on certain days are annually commemorated in the Church, that the recurrence of this festival may preserve the recollection of things so important and salutary. If you ask, then, why we keep the passover, it is because Christ was then sacrificed for us. If you ask why we do not retain the Jewish ceremonies, it is because they prefigured future realities which we commemorate as past; and the difference between the future and the past is seen in the different words we use for them. Of this we have already said enough.
Source: Reply to Faustus the Manichaean (New Advent)