The first Mitzva commanded to Bnai Yisrael was to sanctify the new moon, as it says in this week’s Parsha, החדש הזה לכם ראש חדשים, “This month shall be for you the first month” (12:2). Why were Bnai Yisrael instructed to count months based on the moon, not based on the sun as many cultures do?
The Sefat Emet explains that other nations count by the sun because they only function in daylight, when things are going well for them. However, when it is dark and things begin to go bad for them, they are destroyed and suddenly become nothing. This is not the case with Bnai Yisrael. Even when things turn bad, Bnai Yisrael are still strong and full of light, just as the moon is strong and gives off light at night.
ילקוט מרגליות offers two different explanations why we count by the moon. First, it gives the people a chance to look up to Hashem every thirty days. Second, the relationship between the moon and the sun is likened to the relationship between us and Hashem. Just like the moon reflects the light of the sun, so too Bnai Yisrael reflect the Glory of Hashem. As it says in Tehillim, כי שמש ומגן ה' אלוקים חן וכבוד יתן ה', “For Hashem is a sun and a shield, Hashem gives favor and glory” (84:12).
The Midrash gives the well-known answer that the Jewish Nation is compared to the moon. Just as the moon gets smaller each day until one cannot see it and then gets bigger again, so too Bnai Yisrael get smaller and larger. It might seem that Bnai Yisrael are getting smaller or weaker, but just when they seem to disappear they suddenly get bigger and stronger. Similarly, just like the moon will be as strong and as bright as the sun, as we say in Kiddush Levana, ויהי אור הלבנה כאור החמה, so too Bnai Yisrael will one day be a powerful and strong nation. May we be זוכה to see the fulfillment of this במהרה בימינו.