I. The giving of the Law by Moses prepared the way for the outpouring of the Holy Ghost
The hearts of all Catholics, beloved, realize that today's solemnity is to be honoured as one of the chief feasts, nor is there any doubt that great respect is due to this day, which the Holy Spirit has hallowed by the miracle of His most excellent gift. For from the day on which the Lord ascended up above all heavenly heights to sit down at God the Father's right hand, this is the tenth which has shone, and the fiftieth from His Resurrection, being the very day on which it began, and containing in itself great revelations of mysteries both new and old, by which it is most manifestly revealed that Grace was fore-announced through the Law and the Law fulfilled through Grace.
For as of old, when the Hebrew nation were released from the Egyptians, on the fiftieth day after the sacrificing of the lamb the Law was given on Mount Sinai, so after the suffering of Christ, wherein the true Lamb of God was slain, on the fiftieth day from His Resurrection, the Holy Ghost came down upon the Apostles and the multitude of believers, so that the earnest Christian may easily perceive that the beginnings of the Old Testament were preparatory to the beginnings of the Gospel, and that the second covenant was founded by the same Spirit that had instituted the first.
Source: Sermons (New Advent)