"Hashem said to Moshe and Aharon in the Land of Egypt, 'This month shall be for you the beginning of the months; it shall be for you the first of the months of the year.' " (Shemot, 12:1-2)
The first commandment given by G-d to the Jewish People was to establish the beginning of the Month of Nisan (which wasn't yet called "Nisan," but, rather, the First Month). Until now, the Calendar was, so to speak, in G-d's hands. From this point on, the Hebrew Calendar was placed in the hands of the Jewish People.
Nisan and Rosh Chodesh Nisan - In Relation to Other Months of the Year
1 Nisan (30 days - Rosh Chodesh always 1 day)
2 Iyar (29 days)
12 Adar (29 days - "Adar" preceding Nisan (I or II) always only 29)
This day was chosen by Hashem as the day of the Inauguration of the Mishkan, the temporary Temple which was used in the desert and in the Land of Israel, before the building by Shlomo HaMelech, King Solomon, of the first Bais HaMikdash.
The Midrash discusses the Inauguration of the Mishkan in BaMidbar Rabba, Chapter 13, as follows:
"Rabbi Yossi used to say, 'When did the Inauguration of the Mishkan occur? It began on the twenty third of Adar, and concluded on the first of Nisan. And on all of the first seven days, Moshe used to set up the Mishkan and take it apart at the end of the day. On the eighth day, he set it up, and did not take it apart. And that eighth day was Shabbat, and it was the Rosh Chodesh of Nisan.
On that day, Aharon and his sons arose and washed their hands and feet from the sacred fountain, they worshipped the Divine Service in proper order. On that very day, the People of Israel made vows and donations, sin-offerings and guilt-offerings, brought the first-born of their flocks, and their tithes.
Regarding that day it is said (in "Shir HaShirim," the Song of Songs) " 'Awake, O North Wind, and Come, O South Wind, Perfume my Garden, let its Spices Flow, May my Beloved come to His Garden, and eat of its Delightful Fruit - I have come to My Garden, My Sister, My Bride' - this was the Eighth Day."
Eight "Parshiyot," or Sections of the Torah
On this day, eight "Parshiyot," Sections, of the Torah were taught by Hashem to Israel through Moshe, as follows:
1. "Parshat Kohanim," the Priestly Section; namely Parshat Emor, parts of which were directed specifically towards the Priests, with special restrictions they had to observe
2. "Parshat HaLeviim," the Section for the Tribe of Levi, beginning "And He (Hashem) took the Leviim from the midst of the Children of Israel," because they were needed this day to sing over the "Korbanos," the Sacrifices
3. "Parshat Temaim," the Section for the Ritually Impure, who were informed that they had a "second chance" to bring the Pesach Sacrifice, a month later, because the ritually impure could not participate in the offering of the Pesach Sacrifice
4. "Parshat Shiluach Temaim," the Section defining the camps in which the Children of Israel lived in the Desert, including a special living section for the Ritually Impure, until their Purification
5. "Parshat Acharei Mot Shenei Bnei Aharon," the Section which describes the death of Aharon's two eldest sons on the Day of the Inauguration by a "fire which came forth from G-d." Many commentators have offered explanations for these deaths, but they remain essentially a mystery
6. "Parshat Sh'tuyei Yayin," the Section which forbids a Priest from entering the Mishkan while under the influence of wine or other alcoholic beverage
7. "Parshat Nerot," the Section which describes the Priestly responsibility for maintaining the Menorah in the Mishkan and later, in the Temple, and its flames
8. "Parshat Parah Adumah," the Section dealing with the Red Heifer, or Cow, the sprinkling of whose ashes was a major part of the Purification Ritual
Rosh HaShanah for Five Matters
The First Day of Nisan is Rosh HaShanah for the following five matters:
1. Kings of Israel - They count their reigns from the First of Nisan, such that even if a King began his reign at the end of Adar, once Nisan began, it would be considered as the Second Year of his reign
2. Pilgrim Festivals - The Festival which occurs in Nisan, namely Pesach, is considered the First of the Three Pilgrim Festivals: Pesach, Shavuot, Sukkot. The application is that if a person donated a sacrifice, he has violated the prohibition of "Do not Delay" only if the time interval has passed defined by the three Festivals in the above order, such that if the donation was made, say, after Pesach, "Do not Delay" has not been violated until Shavuot, Sukkot, Pesach, Shavuot, and again, Sukkot, have passed.
3. Months - Nisan is considered the first of the months. The Torah refers to other months as second, third,…twelfth with reference to Nisan.
4. Leap Years - The Court may proclaim a "Leap Year" only until the first of Nisan. Once that date has arrived, the time for "Leaping" has "Leapt"
5. Donation of Shekalim - All Communal Sacrifices brought from this day forward are paid from the shekalim collected in the Current Year; last year's shekalim are no longer used for this purpose.