The month of Abib is the month where the equinox (Where the day and night are equal in length) falls on or before the 1st day of the 1st month (which occurs around March 19th – 21st upon the Gregorian calendar) .
The commandment to keep the feast “at the time appointed in the month Abib [Nisan]” (Exodus 23:15), means that the calendar must keep Abib as close as possible to spring (today we call it the “Vernal Equinox”), and then the moed of Passover can happen. Thus, the month of Abib is the month when Passover falls on or after the Vernal Equinox.
2 “This month shall be the beginning of months to you; it is to be the first month of the year to you.
3 Moses said to the people, “Remember [solemnly observe and commemorate] this day on which you came out of Egypt, out of the house of bondage and slavery; for by a strong and powerful hand YHWH brought you out of this place. And nothing leavened shall be eaten. 4 On this day in the month Abib, you are about to go onward.
7 Unleavened bread shall be eaten throughout the seven days; no leavened bread shall be seen with you, nor shall there be leaven within the borders of your territory. 8 You shall explain this to your son on that day, saying, ‘It is because of what YHWH did for me when I came out of Egypt.’ 9 It shall serve as a sign to you on your hand (arm), and as a reminder on your forehead, so that the instruction (law) of YHWH may be in your mouth; for with a strong and powerful hand YHWH brought you out of Egypt. 10 Therefore, you shall keep this ordinance at this time from year to year.
2 “The sons of Israel are to keep the Passover at its appointed time.3 On the fourteenth day of this month at twilight, you shall keep it at its appointed time; according to all its statutes and ordinances you shall keep it.”
The spring equinox currently occurs each year around March 19th – 21st upon the Gregorian calendar, and is the time when day and night are of approximately equal length. This is a solar phenomenon not a lunar one, hence why it is called a luni-solar calendar.
Again. It is the Light which tells us His appointed times, the days and the years.
14 Then Elohim said, “Let there be light-bearers (sun, moon, stars) in the expanse of the heavens to separate the day from the night, and let them be useful for signs , and for marking appointed times, and for days, and for YEARS;
Some may wonder why it matters when Abib occurs. One reason why Abib should be assigned to the first qualifying month relative to the equinox is that there needs to be a consistency between years so that crops will be in approximately the same stage of development at the same time each year. The importance of the maturity of grain for the wave-sheaf offering (Lev. 23:9-14) is obvious.