Rosh Chodesh Nisan
"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.(Much of the following material regarding Rosh Chodesh Nisan is adapted from Mesorah Publications Stone Edition of the Chumash and from Rabbi Eliyahu KiTov's Sefer HaTodaah).
1 Nisan (30 days - Rosh Chodesh always 1 day)
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."
On this day, eight "Parshiyot," Sections, of the Torah were taught by Hashem to Israel through Moshe, as follows:
The First Day of Nisan is Rosh HaShanah for the following five matters: