This being the 1st of Nissan, a special Torah reading (Exodus 12:1–20), called Hachodesh (“the month,” literally, meaning “the new [moon]”), is added to the regular Torah reading. This is the fourth of four supplementary readings added in the weeks preceding Purim and Passover (Shekalim, Zachor, Parah and Hachodesh).
Hachodesh contains G‑d’s communication to Moses on the 1st of Nissan, in Egypt, two weeks before the Exodus, in which He conveys to him the first mitzvah commanded to the people of Israel: to set the Jewish calendar in accordance with the monthly birth of the new moon, and to regard Nissan, the month of the Exodus, as “the head of months.”
G‑d spoke to Moses and Aaron in the land of Egypt, saying:
“This chodesh shall be to you the head of the months; it shall be for you the first of the months of the year.”
Also conveyed to Moses is the commandment to bring a “Passover offering” to G‑d while still in the land of Egypt:
On the tenth day of this month, they shall take to them every man a lamb . . . a lamb for each household . . .
You shall keep it until the fourteenth day of the same month; and the entire community of the congregation of Israel shall slaughter it towards evening.
They shall take the blood, and put it on the two sideposts and on the lintel of the houses in which they shall eat it. They shall eat the meat in that night, roasted with fire, along with unleavened bread and bitter herbs . . .
Thus shall you eat it: with your loins girded, your shoes on your feet and your staff in your band. You shall eat it in haste; it is G‑d’s Passover.
The blood shall be to you for a sign upon the houses where you are. When I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.
A seven-day festival beginning on this night (the eve of Nissan 15) should be established for all generations.
For on this very day I have brought your hosts out of the land of Egypt . . .
Seven days shall there be no leaven found in your houses . . . in all your habitations shall you eat matzot (unleavened bread).