Moshe warned Paraoh that one final, terrible plague was coming. HaShem would cause all of the firstborn children of the Egyptians to die.
But before HaShem brought this tenth and final plague, He gave the Jewish people a few very important mitzvot (commandments). First of all, they would need to be able to figure out when each new month began. They learned how to determine when each month started by looking at the shape of the moon. Witnesses would report to the Bet Din (Jewish court of law) when they saw a tiny little sliver of a new moon appear in the sky and Bet Din would proclaim that day to be Rosh Chodesh, the first of the month.
It would soon be very important for them to know how to calculate the months of the year, because they were about to be given their first holiday to celebrate -- Pesach (Passover) -- in the month of Nissan, the first month of the year.
