Deuteronomy 16:1

Observe the month of Abib,.... Sometimes called Nisan; it answered to part, of our March, and part of April; it was an observable month, to be taken notice of; it was called Abib, from the corn then appearing in ear, and beginning to ripen, and all things being in their verdure; the Septuagint calls it the month of new fruit; it was appointed the first of the months for ecclesiastic things, and was the month in which the Israelites went out of Egypt, and the first passover was kept in it, and therefore deserving of regard; see Exodus 12:2

for in the month of Abib the Lord thy God brought thee forth out of Egypt by night; for though they did not set out until morning, when it was day light, and are said to come out in the day, yet it was in the night the Lord did wonders for them, as Onkelos paraphrases this clause; that he smote all the firstborn in Egypt, and passed over the houses of the Israelites, the door posts being sprinkled with the blood of the passover lamb slain that night, and therefore was a night much to be observed; and it was in the night Pharaoh arose and gave them leave to go; and from that time they were no more under his power, and from thence may be reckoned their coming out of bondage; see

Exodus 12:12.