In the third month, when the children of Israel were gone forth out of the land of Egypt, the same day came they into the wilderness of Sinai.
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Augustine of Hippo
AD 430
But the fiftyday period is also praised in Scripture, not only in the Gospel, because the Holy Spirit came on the fiftieth day, but even in the Old Testament. Therein fifty days are numbered from the celebration of the pasch by the killing of a lamb to the day on which the law was given on Mt. Sinai to the servant of God, Moses.
We come now to the desert, where the law was given, and this is what Scripture says: “But in the third month from when the people were brought out of Egypt.” The Lord spoke to Moses that those who were going to receive the law should purify themselves in readiness for the third day, on which the law was to be given. So at the beginning of the third month a purification is commanded in readiness for the third day. ..
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. This 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. Against Faustus, a Manichaean
This day. The same on which they departed from the Raphidim, or on the third day of the third month; though St. Augustine understands the first of the month; (Calmet) on which last supposition, allowing 16 days of the month Nisan, 30 of Jiar, and 4 of Sivan, the law was given 50 days after the liberation of the Jews, as the new law was promulgated on Whit-Sunday, on the day of Pentecost. (St. Augustine, ep. 119. 16.) (Worthington)