In keeping Passover (1Co 5:8) JEHOVAH tells us to observe the month in which it occurs as the the first one, the start of a new year (Ex 12:2); He calls it Abib (De 16:1), from a word meaning tender or green, in reference to unripened grain.
One obvious way to observe this month is to note we’re required to find a spotless lamb (Ex 12:5), so we’re to be observing lambs, noting blemishes and defects, looking for that perfect specimen.
On the tenth of Abib we must choose a spotless lamb and set it apart, keeping it four days and identifying ourselves with it, then killing it (Ex 12:6) unto JEHOVAH (De 16:2) and consuming it. (Ex 12:8) Our particular sacrificial lamb is to become part of us forever.
As in each of JEHOVAH’s feasts, here again we have a picture of how we discover Jesus Christ and make Him part of our lives: searching Him out, that perfect specimen of humanity, considering Him and comparing Him with others. Finding Him flawless and divine, we receive Him into our midst (Jn 1:12), studying Him and centering our lives around Him. (1Pe 2:21) Then we see Him on God’s altar becoming our sin (2Co 5:21) and taking it away (Jn 1:29), and trusting Him to reconcile us to God (2Co 5:19) we enter into His rest (He 4:10), identifying with Him and becoming one with Him. (Jn 17:21)
Each Spring in the month of Abib, as new life springs forth in the fields and flocks, we consider anew our Savior (He 3:1), pondering the unsearchable riches of Jesus Christ (Ep 3:8), feeding in the majesty, remembering the day He became our Passover (1Co 5:7), the day JEHOVAH delivered us from the kingdom of this world. (De 16:3)
Naming the first month Abib implies it occurs as grain begins to ripen, placing the feast of First Fruits shortly after Passover. In considering this, I cannot see how a definition for the annual cycle which might place Passover well before or after the Spring harvest in Israel, such as using the Vernal Equinox, could be valid.
Without a temple we cannot keep the physical details about the lamb very well, but when the temple is re-built we will. Until then, we can at least be pondering and appreciating all the spiritual realities the physical rituals imply.