In the early phases of Moshe and Ahron’s mission, they were God’s agents to Pharaoh. But at some point, they had to become agents of the Jewish people as well. That is the point of the first mitzva – Rosh Chodesh, the New Moon.
Rav Hirsch teaches the deep symbolism that belies the mitzva, far beyond a calculation of the calendar dates.
Rosh Chodesh literally means “beginning of renewals”. There were signs and miracles to try and persuade the Egyptians, and there would be a perpetual sign for the Jewish people as well. Rosh Chodesh was to be the recurring sign that would call for ever fresh rejuvenation out of the night and darkness, immunising the people from the corruption they’d find themselves immersed in, from Egypt to everywhere else.
The procedure for calling it is human-centric – it requires multiple witnesses, and multiple judges to form a court. For simple declarations, one of each is enough, but more is required for cases concerning relationships. Rosh Chodesh is not an astronomical phenomenon; it is solely dependent on human criteria. It is the court as representatives of the Jewish people that decide when it is or is not Rosh Chodesh.
The Chagim are all based on when Rosh Chodesh is. Rosh Chodesh is called a מועד, which means a designated meeting time. The מועדים are designated times for a meeting between God and the Jewish people. The meeting is voluntary between both sides, which is the timing is only general, with latitude on our part; the meeting will be by mutual choice.
It is for this reason that this is the first mitzva communicated to the Jewish people as a whole; the mitzva that binds the relationship between the Jewish people, Moshe, and God.
The natural phenomena are not the reason. Rather, as each time the moon reunites with the sun, receiving new light, the Jewish people too can find their way back, no matter where they may be, or what darkness they find themselves in. The natural phenomena are the symbol.