Explanation of RAMBAN
Rabbi Moshe ben Nachman, a great thirteenth century Jewish scholar of Spain, later of Israel, writes in his commentary to the Five Books of Moses, in explanation of the above verses, Shemot,12:1-2, as follows:
"The verses mean that this month should be counted first. And beginning with it, should the count proceed to the second, the third, and so on, till the end of the sequence of months with the twelfth month. For the purpose that this month should be a commemoration of the Great Miracle. For every time we mention the months, the Miracle will be alluded to. It is for that reason that the months do not have names in the Torah, but rather they are identified by number…"
"And it is similar to the way that days are referenced with reference to the Day of Shabbat; for example, the First Day of Shabbat (for Sunday), and the Second Day of Shabbat (for Monday), as I will explain in the future…Thus, when we call the Month of Nisan "the first" and Tishrei "the seventh," the meaning is the first with reference to the Redemption and the seventh with reference to it…"
"And our Rabbis have mentioned this matter, and they said that the names of the months came back with us from Bavel. For originally they had no "names" for us, and the reason is that they were "in commemoration of the Exodus from Egypt." But when we returned from Bavel, and the prophecy of "it will no longer be said 'by the Life of Hashem, who took the Jewish People out of Egypt,' rather it will be said 'by the life of Hashem, who raised up and brought the People of Israel from the Northern Land' (Yirmiyahu 16:14-15) was fulfilled, we changed our practice and began to call the months by the names which were used in those lands, as a reminder that we had been there, and that it was from there that Hashem took us out…"
"For these names, Nisan, Iyar and the rest are Persian names, and appear only in the Books of the Prophets who prophesied in Bavel, and in Megilas Esther… And still today, the non-Jews in those lands use the names Nisan and Tishrei, etc. as we do. And thus we are following the same practice with reference to the second redemption as we did in connection with the first."
Names of the Month
This month has three names:
"Rosh Chodoshim," or "HaChodesh HaRishon," the First Month
"Chodesh HaAviv," - The Spring-time Month
1) It is called the "First Month," because it is the Month of Redemption, the month of the Exodus of the Jewish People from slavery in Egypt, the House of Bondage. The Society of Egypt was built on the institution of Slavery. Slaves were used to build treasure cities, such as Pitom and Raamses (Shemot 1:11), and the Pyramids, giant tombs for the Pharaohs (possibly hinted at when the Jewish People complained to Moshe, "Are there not enough graves in Egypt that you had to take us out to die in the desert?" (Shemot 14:11))
No individual slave, nor group of slaves, and certainly no enslaved nation had ever escaped from slavery in Egypt.
2) It is called the "Chodesh HaAviv," the Spring-time Month, because the Hebrew Calendar is set up so that the month must fall in the Spring. This is to satisfy the G-dly requirement whereby Hashem said to the Jewish People, "You are leaving today, in the Month of Spring-time!"
This is accomplished by having a calendar which is basically "lunar;" that is, dependent on the revolution of the moon around the earth (once every twenty-nine and a half days, approximately), but adjusted in a "shanah me'uberet," a "leap year" (literally, a "pregnant" year), by adding an additional month of Adar. If not for the adjustment, Nisan, with Pesach, would travel through the seasons of the year, much as Ramadan, the Moslem Holy Month, based on the totally lunar Moslem Calendar, migrates through the seasons.
3) The Name "Nisan" is of Babylonian-Persian origin, as are the names of all the twelve months of the Hebrew Calendar. But it also has the suggestion of Spring-time, or blossoming, because the similar word, "nitzan," in Hebrew, means a blossom.
Zodiac Sign of the Month
The Zodiac sign of Nisan is the "kid," the young goat, that animal which was worshipped in Egypt, but which the Jewish People were commanded to sacrifice as the Pesach sacrifice.