The narrative leading up to the 10th plague of the firstborn is interrupted by commandments that deal with the relationship between God- Hashem and his people - (a) the Jewish calendar= the sanctification of the New Moon- Rosh Chodesh and (b) the laws of Pesach- Passover. The timing of the laws of Pesach is obvious, but why was the calendar-the sanctification of the New Moon given now. And also why was' kidush ha'chodesh' – the sanctification of the new moon given such prominence as being the first commandment given by God to the Israelites as a nation.
An insight and understanding of the Oral laws concerning the sanctification of the New Moon will answer our question - see R' SR Hirsch, Timeless Hirsch R' Adlerstein. The Jewish calendar is based on the moon and regulated by the sun, so that the month of Nissan and the holiday of Pesach –Passover will fall in the spring. Unlike other calendars which are absolutely predictable as they follow the path of the sun or moon, the Jewish calendar is based less on ' calculation ' and more on the actual visual sighting of the moon and when the Beit Din's proclaims and sanctifies the New Moon. So in a case where the witnesses said that they sighted the beginning and renewal of the new moon on Monday night= Tuesday and it is known that this sighting was viewed by lots of other people, but Beit Din only managed to perform the declaration and sanctification ceremony on Wednesday, Wednesday is Rosh Chodesh, the first day of the new month, and not Tuesday, the day of the sighting of the new moon. The Beit Din can manipulate Rosh Chodesh- the first day of the new month, so that Shabbat does not come before or after Yom Kippur, as it would cause hardship for people. The declaration of the court has finality, even when it is later learned that the testimony of the witnesses was inaccurate, whether by error or intentionally. It was important for communities to know when Rosh chodesh fell so as to know on which day the holy holiday would fall. Distant communities , because of this lack of predictability and they did not yet hear which day the court declared as Rosh chodesh, would have to observe an extra day , the second day of the holiday in the diaspora , not one day because of this doubt. Because of the exile and that there were longer judges who were properly ordinated and had semicha needed for the sanctification of the new moon process, Hillel the second, instituted a fixed calendar. What remains of the authentic Jewish calendar is that communities outside of Israel observe 2 days of holidays-- the 2nd day of the diaspora and don't rely on the fixed calendar.
The downplaying of the astronomical calculation and the fact that the witnesses and the Beit Din process ' trumps' following ' nature and its laws' means that man has become master over time and is able to transcend time. While slaves in Egypt the Israelites were not owners of their time, it was their masters. When they became free, there was still the danger of becoming a ' slave of time' and nature and not being the ones who control time, rather time controlling them. A fixed calendar, following predictable astronomical events and linking the holidays to them would give the impression that we were worshiping the dutiful periodicity of Nature, something glorified by the pagan Egyptian culture. It would imply that the relationship between man and God is fixed, static and constrained. It is the ' oral law' that gives man a role, collaborating with God in the on-going process of creation and elevating nature making Judaism a dynamic religion which can deal with the challenges of time and yet remain authentic to the Torah given at Mount Sinai and not become a fossilized religion. The waxing and waning of the moon and its renewal are merely symbols for us , like the rainbow in Noah's time, modeling the rhythm of life and encouraging us to rejuvenate ourselves and renew our dedication to God. ' It is not the astronomical first appearance of the moon , the Rosh chodesh that has us count the days till the holiday , the mo'ed , the special days that we meet and encounter God. It is the renewal and newness that takes place within ourselves that allows us to spend time with God on the special days of the year. Because we can change and move towards Him, we experience something powerful when He makes Himself available to us. Without that change, we would be commemorating the past, and it would not be a moe'd, a meeting and encounter. In other words, Rosh Chodesh is not determined by the moon as by the way it is noticed and perceived by us. It is man that declares the new moon, not the moon itself. Although today we use a fixed calendar , the second day of Yom Tov- holiday keeps alive the spirit of sanctification of the new moon by the Beit Din –law court '- R' Adlerstein.
.It is the so-called inefficient process that consists of visually sighting the moon and proclaiming and sanctifying the new moon that gives expression to a value of being a master over time and nature. In contrast , the fixed calendar with its astronomical accuracy makes us slaves of time and nature.This lesson was crucial to the Israelites achieving true freedom and not becoming slaves of time and nature.