Abib was the first month of the Jewish calendar. The Passover began towards the end of the fourteenth day of the first month.
Exodus 34:18 "You must celebrate the Festival of Unleavened Bread. For seven days the bread you eat must be made without yeast, just as I commanded you. Celebrate this festival annually at the appointed time in early spring, in the month of Abib, for that is the anniversary of your departure from Egypt.
Leviticus 23:5 In the fourteenth day of the first month at even is the LORD’S passover. 6 And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD: seven days ye must eat unleavened bread.
Deuteronomy 16:1 Observe the month of Abib, and keep the passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night.
The name of the month was changed to Nisan - Meaning: ‘their flight’ - after the Babylonian Captivity (Neh. 2:1).
Abib/Nisan falls somewhere within our months of March and April. It changes in relation to our months because the Jews use a lunar calendar (months of 29 or 30 days), with an extra month inserted in the year periodically to maintain the seasons.
I hope you have found this helpful.