In Parshat Bo, Shmot 12:1-2 we learn about Rosh Chodesh: “God said to Moshe and Aharon in the land of Egypt: This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.”
Rashi comments: God showed him the moon at its renewal, and He said to him, “When the moon renews itself, let that be for you the beginning of a new month.” God was specifically referring to the month Nisan: This month (Nisan) shall be the beginning of the order of the counting of the months, so that Iyar shall be called the second month and Sivan the third.
Ramban points out that Nisan is the first month from the redemption and Tishrei is the seventh. This is the reason for, “it shall be the first…to you,” for it is not the first in the year, but it is the first for you, for such is it called as a remembrance of our redemption.
Ramban adds that our Sages have already mentioned this topic in the Talmud, Rosh Hashanah 6a, and said that the names of the months came with us from Babylonia, for at the start our months had no names. The reason for this is that at the start the order of the months was as a remembrance of the Exodus from Egypt, but when we left Babylonia and the verse was fulfilled (Yirmiyahu 16:14-15), “Behold days are coming – the word of God – when it will no longer be said, ‘As God lives, Who took B’nai Yisrael out of the land of Egypt,’ but rather, ‘As God lives, Who brought B’nai Yisrael from the land of the North and from all the lands where He had scattered them’; and I shall return them to their land, which I gave to their forefathers.”
Ramban explains: We returned to calling the months by the names by which they were called in Babylonia, as a reminder that there we stood and from there God drew us out. The names Nisan, Iyar, and the others are Persian names and are only found in the books of the Babylonian prophets (Zechariah 1:7, Ezra 6:15, Nehemiah 1:1) and in Megilat Esther (3:7). Therefore the verse says, “In the first month, which is the month Nisan,” similar to, “They cast a pur, that is, the lot.” And until today the nations in the lands of Persia and Media call the months Nisan and Tishrei and all the rest like us. And these names recall through the months the second redemption just as we did until now for the first one.
According to Sforno, from now on, these months will be yours, to do with as you like. You have My authority to organize your own calendar. This is in contrast to the years when you were enslaved when you had no control over your time or timetable at all. While you were enslaved, your days, hours and even minutes, were always at the beck and call of your taskmasters.
On each Rosh Chodesh, as the moon renews itself, we have a new chance to set up how we want to live our lives. We can step back to see what is important, reflect on the past redemptions and pray for a brighter future.