First Mitzva commanded to Bnei Yisrael while we were still in Egypt - even before Korban Pesach. (Had to be before Korban Pesach, because there is no Pesach without Rosh Chodesh Nisan.)
Hopefully, no matter how many pieces you read about the Jewish Calendar, you will always find something new to know and/or to ponder.
Warning: The next many paragraphs might be a bit confusing. Please don't worry. Keep reading. Or, if you must, skip to p.65 "And here's the point!"
We've just begun the month of Sh'vat, so let's use it as an example (things don't always work out this way).
Last Shabbat, during Birkat HaChodesh, the molad of Sh'vat was announced.
[For today's calendar, that is a good match but it is somewhat coincidental. The R"Ch of each month is not determined by the molad of the month. Only the molad of each Tishrei is used to fixed Rosh HaShana according to specific rules.]
Something else you should know. The molad is not the birth of the new moon, it is not the time we can see the tiny sliver of moon that is visible at the beginning of the Jewish month. The real molad is the instant that the Moon is between the Earth and the Sun. The Moon is totally invisible to us at that moment and for many hours thereafter.
We also should know that the molad we announce - according to our tradition is not the real moment. The announced molad is based on a calculation using the AVERAGE time it takes the Moon to cycle through its phases. That period of time is called a lunation and averages 29d 12h 44m 1p. (1 part is 3 and a third seconds; 18 parts or chalakim in a minute; 1080p in an hour.)
The molad we announce is called the MOLAD EMTZA'I. The real molad is called the MOLAD AMITI, actual molad or the astronomical molad (a.k.a. New Moon).
A little more detail before we get to the point.
Our current in use calendar uses average molad and 'ignores' the actual molad, which can be several hours before or after the average. This is a result of the laws of planetary motion (enough said on that here).
When we had a Sanhedrin AND IYH when we will once again have one, then the average molad is not relevant, but the actual molad is used. It is used (past and future) to determine when and if the first visibility of the lunar crescent will be visible to the naked eye during early evening following the 29th day of a month.
In our example (last Sunday), the actual molad for Sh'vat 5779 was 3:27am on Sunday, January 6th.
A sitting Sanhedrin had/will have a committee of judges who were experts in astronomy (or have access to the good websites who do the math for them). They would have determined that the Moon would not have been visible on Sunday night. Monday, then, would have been the 30th of Tevet (which we do not have in our fixed calendar), and Tuesday (not Monday) would have been Rosh Chodesh Sh'vat.
And here's the point!
Without a Sanhedrin, with the calculation method of determining our Calendar, Rosh Chodesh Sh'vat 5779 is exactly where HKB"H originally intended it to be. With a Sanhedrin, it would have been a day later.
Which method of making the calendar does HKB"H prefer?
Not the one that uses the calculations and the average molads.
And the reason WHY is the whole point of this Lead Tidbit. We can say it with one word. LACHEM! The word that HKB"H used twice in the pasuk that commanded our Calendar. He (so to speak) is thrilled and delighted to wait until we are able to see the newness of the Moon and are able to declare the day holy as active (junior) partners with G-d in the process of Sanctifying Time. We are only temporarily relegated to being passive bystanders to the process. Our calendar - by either method - is precious and holy. But the main command was to Moshe and Aharon and the Sanhedrins throughout the generations. G-d keeps us (so to speak) at arm's distance until we merit the Sanhedrin once more, BIMHEIRA B'YAMEINU AMEN.