In Parshat Bo, the Pasuk states, “HaChodesh HaZeh Lachem Rosh Chodashim,” “This month shall be for you the beginning of the months” (Shemot 12:2). This Pasuk refers to the Mitzvah of Rosh Chodesh. Hashem commanded Moshe and Aharon that this month (Nissan) should be the first of all months. Rashi explains that the commandment is not just to announce the new month, but also to sanctify the new moon.
Many Rabbis see special qualities of the moon and demonstrate specifically how special it is to the Jewish people. In the Talmud Yerushalmi (Masechet Rosh Hashanah), Rabi Yochanan asks why, if only the sun was created for light, was the moon created? He answers by quoting a Pasuk from Tehillim, “Asah Yareiach LaMoadim,” “He made a moon for the festivals” (104:19). The moon’s purpose is to show us when the holidays fall. Hashem told Moshe and Aharon to set up a calendar based on the moon in order to sanctify the moon. Through this calendar, we know when all the holidays will be, based on the light of the moon. We learn from this the amazing way the Jewish calendar is set up.
Hashem commanded us that Pesach must always be in the springtime. Therefore, the calendar is set up so that Pesach is always in the spring. Since the lunar calendar is approximately 11 days shorter than the solar calendar, a leap year (with an extra Adar) is occasionally needed to ensure that Pesach is always in the spring and that Sukkot is always in the fall. This is one of Hashem’s great miracles - the moon and the Jewish calendar.
Those who diligently study the intricate workings of the calendar feel a profound sense of awe for Hillel and his court (those who established the calendar). Everything is so precise; every possible flaw is compensated for. This is the wisdom of the great Torah Sages.