Sanctification of the New Moon
The first mitzva in the Book of Shemot, is referred to as קידוש החודש, sanctifying the month. It is only the fourth commandment in the Torah. The Book of Bereishit only had the Mitzvot of “Being fruitful and multiplying,” circumcision, and גיד הנשה, the prohibition of eating the sciatic vein.
Rav Kook pointed out that the process of declaring the moon, had three aspects to it. There was חוש, the use of the senses, שכל, the use of the intellect, and רוח הקודש, Divine inspiration.
The sensory aspect of the Mitzva, was the witnessing of the first slivers of the moon. The first to notice the change in the horizon, needed to testify as witnesses in front of the Great Sanhedrin. The intellect was used to examine the witnesses to be certain that what they saw was actually the New Moon, and not a cloud.
The Divine Spirit was involved in what was known as סוד העיבור, the secret to the additions to the month. This required great Divinely inspired wisdom to know how to establish whether the month was twenty-nine or thirty days.
The Rambam gives a very complicated mathematical formula, where one is able to pinpoint the exact location on the horizon, where the moon is due to appear. He wrote that the use of the formula works forever. This is how the judges knew if the witnesses were telling the truth.
This system of reporting the New Moon and establishing Rosh Chodesh, existed until the Sanhedrin disbanded in the year 358 of the common era. This was when the Sanhedrin was situated in Tiberius.
After this, a set calendar was implemented with seven leap years during each nineteen year cycle. And this is why our English and Hebrew birthdays coincide every nineteen years.
Most people do not realize the intricacies and details involved in the Sanctification of the Month. It is one of the more fascinating Mitzvot in the Torah.