This month shall be to you the head of the months; to you it shall be the first of the months of the year. (Exodus 12:2)
The first mitzvah (command) given by the Eternal One to Moshe and Aharon was to start counting the beginning of the year from the month in which Chag HaMatzot (Passover) is held.
Today you are going out, in the month of aviv. (Exodus 13:4)
We are commanded to begin the Biblical year in the spring and not in the fall (“Rosh HaShannah”) as the Talmud teaches.
Aviv refers to the stage of the barley crops in Eretz Yisrael (Land of Israel). Aviv refers to the mature stage of the barley crops – a time when the barley becomes more brittle and turns a yellowish color.
Chag HaMatzot is a Biblical festival that begins on 15 Nisan (in March or April). Chag HaMatzot is an seven-day holiday that celebrates the Exodus of the Hebrews from Egypt and our freedom from oppression in modern days. The holiday is also referred to as Chag HaAviv (the Spring Festival). It is the first of the three pilgrimage festivals with both historical and agricultural significance (the other two are Shavuot and Sukkot). Agriculturally, it represents the beginning of the harvest season in Israel.