Exode 12:2

This month is ordained to be to you the beginning of the months; and from it you shall begin to number for festivals, and times, and cycles; it shall be to you the first of the number of the months of the year.

THIS MONTH SHALL BE UNTO YOU THE BEGINNING OF MONTHS. This is the first commandment which the Holy One, blessed be He, commanded Israel through Moses. Therefore it says here [that the Eternal spoke unto Moses and Aaron] in the land of Egypt, for the rest of the commandments of the Torah were given to him on Mount Sinai. It may be that the intent of the expression, in the land of Egypt, is to exclude the city of Egypt, just as our Rabbis have said: “In the land of Egypt. This means outside the city.”
Now Scripture should have first said, Speak ye unto all the congregation of Israel, saying: This month shall be unto you the beginning of months, and so on to the end of the chapter. [Why then is the verse, Speak ye, etc., mentioned after the verse, This month, etc.?] It is because Moses and Aaron — [as mentioned in Verse 1: And the Eternal spoke unto Moses and Aaron in the land of Egypt, saying: This month, etc.] — are in the place of Israel. Saying it to them is equivalent to saying it to Israel in all their generations. In the following verse, however, He repeats by saying, Speak ye unto all the congregation of Israel, in order to command them something which is not binding for all time, namely, the buying of the paschal offering in Egypt on the tenth day of Nisan., etc., applying as it does for all time, it is sufficient for Scripture to mention only Moses and Aaron in connection with it, since they are in place of Israel for all times. But since the command mentioned in Verse 3, In the tenth day of this month they shall take to them every man a lamb, applied only to the paschal offering in Egypt, He therefore preceded it again by saying, Speak ye unto all the congregation of Israel, the Israel of that time.
According to the Midrashic interpretation, lachem (unto you) [in the verse, This month shall be unto you], means that “the Sanctification of the New Moon” is to be performed only by a Court of experts [as Moses and Aaron were]. And this is the reason it does not say at the beginning [of Verse 2], Speak ye unto all the congregation of Israel, since “the Sanctification of the New Moon” can be performed only by Moses and Aaron and their like.
Now the purport of the expression, This month shall be unto you the beginning of months, is that Israel is to count this as the first of the months, and from it they are to count all months — second, …

'בארץ מצרים לאמר החדש הזה וגו, according to the view of Rabbi Yehoshua in Rosh Hashanah 11 who holds that the earth was created in the month of Nissan,

לחדשי השנה, “whenever I will say to you that some festival is to occur in the ‘eight’s or ninth month,’ the number refers to Nissan being the ‘first’ month relative to this.” According to Rabbi Eliezer, who holds that the earth was created in Tishrey, we would have to understand the plain meaning of the text as follows:

החודש הזה לכם, even though other nations do not know the institution of “new moon,” you will have such an institution, so that the months can be numbered by ordinal numbers, i.e. “the second, the twelfth,” etc. (compare Esther 3,7) This is the month from which you will begin dating important events in your history/calendar, for this is the month in which you attained your freedom as a people. Whenever the Torah refers to an event by naming the month in which it occurred or would occur, the ordinal number used is relative to this month, the month of Nissan, which rates as the “first” month. This is spelled out even more clearly in Exodus 19,1 בחודש השלישי לצאת בני ישראל ממצרים, “in the third month, after the Exodus of the Children of Israel from Egypt.” The date of the Exodus from Egypt remained uppermost in the way our prophets recorded important historical events, as for instance, in Kings I 6,1 when the completion of Solomon’s Temple is dated as having occurred in the 480th year after the Exodus from Egypt.

החודש הזה לכם ראש חדשים, from now on these months will be yours, to do with as you like. [you have My authority to organise your own calendar. Ed.] This is by way of contrast to the years when you were enslaved when you had no control over your time or timetable at all. [Freedom, i.e. retirement from the “rat race,” means being able to formulate one’s own timetable. Ed.] While you were enslaved, your days, hours, minutes even, were always at the beck and call of your taskmasters.

החודש הזה, “this month. [Seeing that the word “month” is an abstract term that cannot be described as visible, ”the meaning of G–d’s words are; “when you observe the position of the moon in which you see it at this time, this is when the lunar cycle enters a new phase, i.e. when the month in the Jewish calendar begins.” This event is to be sanctified by you on each occasion. The members of the Jewish High Court will have to officially proclaim this event. The word לכם, ”for you,” is G–d’s authority to appoint the High Court as competent to declare when the moon has renewed itself. If the members of the High Court deem it in the national interest, they have the authority to postpone the new moon for an extra day. [They do not have the authority to advance it for a day. Ed.] When Rosh Hashanbah approaches, G–d informs the angels that He will sit in judgment of mankind, and He gave them a preview similar to what He did with Avraham when He was about to destroy Sodom and its satellites. According to Rabbi Aushiyah, after He had told the angels that He was afraid He would have to destroy mankind, it happened once that when the day came and He had not sat in judgment, the angels approached G–d asking Him if He did not say to them: “Tomorrow I’ll sit in judgment over all of mankind and the judgment will result in our allowing mankind to disintegrate”? G–d replied: “My children have forced Me to delay this for a day when I authorised them to decide on making adjustments to the calendar.” [The editor of my edition of this edition of the Daat Z’keynim, points to a similar, but not identical statement in Sh’mot Rabbah 15,2, the prediction of mankind being destroyed being missing. Ed.] [If I understand the point of the Midrash correctly, if G–d had sat in judgment on the day on which Rosh Hashanah should have been according to His calendar, on that day the balance of good and evil on earth would have resulted in mankind having forfeited the right to exist. Due to the Rabbis having decided to observe Rosh Hashanah one day later, the pendulum by then will have swung to a positive balance allowing G–d a chance to continue to give mankind another chance. Ed.] Being aware of this, Moses said in Deuteronomy 4,7:כי מי גוי גדול אשר לו אלוקים קרובים אליו כה' אלוקינו בכל קראנו אליו, “for what great nation is there that has G–d so close to them, as the Lord our G–d whenever we call upon Him?" The word קראנו in that verse is to be understood as if it had been spelled קריאתנו, …