exodus 19:1 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. - Free Bible Online

      Ex 19:1-25. ARRIVAL AT SINAI.

      1. In the third month--according to Jewish usage, the first day of that month--"same day."--It is added, to mark the time more explicitly, that is, forty-five days after Egypt--one day spent on the mount (Ex 19:3), one returning the people's answer (Ex 19:7, 8), three days of preparation, making the whole time fifty days from the first passover to the promulgation of the law. Hence the feast of pentecost, that is, the fiftieth day, was the inauguration of the Old Testament church, and the divine wisdom is apparent in the selection of the same reason for the institution of the New Testament church (Joh 1:17; Ac 2:1).

JFB.