Perhaps the best known of the Jewish festivals, Passover commemorates the Exodus of the Jews from Egypt – the transformation of the Children of Israel from an extended family to a nation, and the transition of that nation from slaves under the Egyptian Pharaoh to a free people serving One God.
The Torah commands us in several places to observe Passover, most notably:
"You shall keep the feast of unleavened bread…for seven days at the appointed time in the month of Aviv, for in that month you came out of Egypt" (Exodus 23:15)
"Observe the month of Aviv, and keep the Passover … for in the month of Aviv the Lord your God brought you out of Egypt by night." (Deuteronomy 16:1)
While there is no month called Aviv, Exodus Chapter 12 refers to the month of Aviv as the first month of the year, which is the lunar month of Nissan.
In modern Hebrew, aviv means the season of spring. But in the Torah, aviv refers to the particular stage of growth in grain at the beginning of its ripening process, when the stalks have hardened. First the barley and then the wheat ripen during this period.
By linking the historical event of the Exodus from Egypt with the agricultural month of Aviv, the Torah binds the lunar calendar of historical events with the agricultural realities of the Land that occur according the solar calendar, thus perpetually binding the Oneness of the Creator who controls history and nature.
For more information on the history and agricultural connections of Passover, please see the book "Nature in Our Biblical Heritage" by the founder of Neot Kedumim, Nogah Hareuveni.