"AND HASHEM SPOKE TO MOSES AND AARON IN THE LAND OF EGYPT, SAYING: 'This month will be for you the beginning of months; it will be the first month of the year to you. Speak to the entire congregation of Israel, saying: In the tenth day of this month every man will take a lamb, according to their fathers' houses, a lamb for each household; and if the household is too little for a lamb, then he and his next door neighbour next will take one according to the number of the souls; according to every man's eating you will make your count for the lamb.
Your lamb should be without blemish, a male of the first year; you will take it from the sheep, or from the goats;
and you will keep it until the fourteenth day of the month; and the whole assembly of the congregation of Israel will kill it at dusk. And they will take the blood, and put it on the two side-posts and on the lintel, on the houses where they will eat it.'" (Exodus 12:1-7)
There had been new moons since the beginning of time, so why make a fuss over it? Why now?
With the words, "This month will be for you the beginning of months" G-d didn't quite recreate the world, but He did make one incredible change: From now on the children of Israel would become partners with G-d in the maintenance of the world. This commandment, the first of many more to follow, was a call into action for the Israelites. From now on, our clocks and calendars would be set according to the testimony of two Jewish witnesses that the new moon had indeed appeared in the heavens. No testimony - G-d forbid! - means no new moon. No new moon means no holidays: no Passover, no Shavuot, No Rosh HaShana, Yom Kippur, Sukkot or Shmini Atzeret. In short, none of the things which bring Israel close to G-d, celebrating His appointed seasons in His Sanctuary, binding heaven to earth.
So it is little wonder, then, that the testimony of the new moon - Rosh Chodesh - would be given before the elders of the Sanhedrin - the High Court of Jewish law, and that the Sanhedrin would come to be located in the very walls of the Courtyards that surrounded the Beit HaMikdash - the Holy Temple. For as junior partners to G-d in the keeping of His creation, our place is next door to His: the Holy of Holies, where His presence dwells.