Or HaChaim on Exodus 12:2/אור החיים על שמות י״ב:ב׳

1 החודש הזה לכם ראש חדשים, "This month shall be for you the beginning of the months." Why did the Torah repeat ראש חדשים and ראשון הוא? I believe the meaning of ראש is that the month of Nissan will be particularly important, the choicest of the months. We find the word ראש used in this sense in Exodus 30,23 when the most important of the spices in the making of oil for anointing is discussed.

2The Torah stressed the word לכם, for you, to tell us that this month would be special only for the Jewish people. Our sages pointed out in Rosh Hashanah 11 that Israel was redeemed from Egypt in Nissan and that the future redemption would again occur in that month. This is so because this month is a harbinger of good tidings for Israel. It is appropriate that Israel should begin the count of the months of the year by making Nissan the first month.

3Another reason that the Torah writes החודש הזה instead of writing חודש זה, is to allude to something or someone we are already familiar with. The reference to this known entity is G'd who is described in 15,2 as זה קלי, "this (One) is my G'd." The Torah alludes to the promise that it is this G'd who will orchestrate the redemption.

4An additional meaning derived from the wording of this line is that Israel will be a ראש, head, as a result of what happened in this month. The numerical value of the word זה is 12, i.e. Nissan will be the first and most distinguished of these twelve months.