Deuteronomy 16:1

שמור את חדש האביב WATCH THE MONTH OF ABIB — This means: Before it comes watch whether it will be capable of producing ripe ears (אביב), so that one may offer the Omer meal-offering during it, and if not (i.e. if you observe that the ears will not be ripe by the 16th of Nisan), then intercalate the year (i.e. add a month to the winter-period, so that the month Abib falls later than it otherwise would, by which time the ears will be ripened) (cf. Sanhedrin 11b and Note 4 on p. 191 of Leviticus in the Silbermann edition of the Pentateuch). ממצרים לילה [FOR IN THE MONTH OF ABIB THE LORD THY GOD BROUGHT THEE FORTH] FROM EGYPT BY NIGHT — But did they not go forth by day , as it is said, (Numbers 30:3) “on the morrow after the Passover the children of Israel went out … [in the sight of all the Egyptians]”? But it states that they went out by night because Pharaoh gave them permission to go forth by night, as it is said, (Exodus 12:31) “And he called for Moses and Aaron by night, [and said, Rise up, go forth from among my people etc.]” (cf. Sifrei Devarim 128:5; Berakhot 9a).

שָׁמוֹר֙ אֶת־חֹ֣דֶשׁ הָאָבִ֔יב וְעָשִׂ֣יתָ פֶּ֔סַח לַיהֹוָ֖ה אֱלֹהֶ֑יךָ כִּ֞י בְּחֹ֣דֶשׁ הָֽאָבִ֗יב הוֹצִ֨יאֲךָ֜ יְהֹוָ֧ה אֱלֹהֶ֛יךָ מִמִּצְרַ֖יִם לָֽיְלָה׃

Observe the month of Aviv, and keep the passover to the Lord thy God: for in the month of Aviv the Lord thy God brought thee forth out of Miżrayim by night.