“This month shall be for you the head of the months. It shall be for you the first of the months of the year.” (Shemot 12:2)
This pasuk introduces the first mitzvah that Hashem revealed to Moshe. We are commanded to establish a calendar. The calendar is to be based on the cycles of the moon. The emergence of the new moon will determine the beginning of each month. The courts are charged with the responsibility of accepting testimony regarding the appearance of the new moon and declaring the new month. Today’s Rabbinic courts do not have the authority to accept this testimony and cannot declare a new month. We determine the date of the new month based upon a calendar developed our Sages.
The first day of the month has some special observances. One of these observances is that the Hallel prayer is recited in the morning service. The Talmud explains that the recitation of the Hallel on Rosh Chodesh – the new month – is not a Rabbinic decree. It is custom – a minhag.
In order to appreciate this observation some background is required. The Sages enacted the practice of reciting the full Hallel on festivals. The Sages established an obligation to recite the complete Hallel on eighteen days of the year. These are, the eight days of Chanuka, the seven days of Sukkot, Shemini Atzeret, the first day of Pesach and on Shavuot. Outside of Israel, the complete Hallel is recited twenty-one days. In addition to the days it is recited in Israel, it is recited on Simchat Torah, the second day of Pesach and the second day of Shavuot.
Although the complete Hallel is recited only on the first day of Pesach – or the first two days, outside of Israel – an abbreviated version of Hallel is recited on the remaining days of the festival. This abbreviated Hallel is also recited on Rosh Chodesh.
Why do we sometimes recite the complete Hallel and on other occasions an abbreviated form? The Talmud explains that the Sages established a Rabbinic obligation to recite the complete Hallel on the eighteen days outlined above. However, the custom developed to recite Hallel on additional occasions. This custom is not part of the original decree of the Sages. In order to identify the occasions on which the recitation of Hallel is customary but not part of the original decree, an abridged Hallel is recited on those occasions that are established through custom. In other words, when Hallel is recited in response to the original decree of the Sages, the complete Hallel is read. When Hallel is recited in response to custom, an abbreviated Hallel is read.
What is the reason for the custom to recite an abridged Hallel on these additional occasions? Tosefot explain that Hallel is recited to recognize a miracle or to celebrate as festival. Based on this criterion, we can easily explain the custom to recite Hallel on all of the days of Pesach. Although the original decree of the Sages only requires that the complete Hallel be recited on the first day of the festival, the custom extends the requirement to the entire festival. The custom is consistent with the original decree and is an extension of this decree. However, the custom to recite an abridged Hallel on Rosh Chodesh is more difficult to understand. Rosh Chodesh is not a festival. What is the basis for this custom?
Aruch HaShulchan offers an interesting explanation of this custom. Before we can review his explanation, an introduction is required. The Sages established a blessing that is recited each month at the appearance of the new moon. This blessing – Birkat HeLevanah – is composed of two themes. The blessing begins with recognition that the renewal of the moon reflects the system of natural laws that Hashem created to govern the universe. We acknowledge the wonders of this system and that the natural laws are an expression of Hashem’s majesty. The blessing then takes up a second them. We compare the cyclical renewal of the moon to the inevitable redemption and renewal of the Jewish people. We declare that the renewal of the moon is symbolic of the eventual salvation of our people.
The blessing can be understood on a deeper level. Most people take for granted the regularity of the physical laws. We go to sleep at night certain that the sun will rise in the morning. We are sure that just as the moon renewed itself this month, so too it will renew itself next month. Yet, it is more difficult to affirm with absolute conviction that we will be redeemed from exile. We are aware of the promises of the Torah that the Jewish people will be redeemed. But our exile has extended over a period of centuries. It seems far less certain than the renewal of the moon and the rising of the sun.
The blessing responds to this confusion and insecurity. It declares that the physical laws operate in conformity with the will of the Creator. Their regularity and consistency is a reflection of His will. So too, our eventual redemption is promised by the Creator. Therefore, the certainty of our redemption is as absolute as the regularity of the physical laws.
Aruch HaShulchan suggests that this blessing identifies a basic theme of Rosh Chodesh. He suggests that Rosh Chodesh is associated with the theme of the redemption of Bnai Yisrael. As we explained above, Hallel is recited on the occasion of a festival or in response to a miracle. The miracle of our redemption has not yet occurred. However, on Rosh Chodesh we acknowledge the inevitability of the miracle of redemption. The recitation of the abbreviated Hallel on Rosh Chodesh is an affirmation of our conviction in the certainty of this future redemption.
Our parasha suggests an alternative explanation of the custom to recite an abridged Hallel on Rosh Chodesh. In order to develop this explanation, we must consider our pasuk more carefully. As we have explained, the courts are charged with the responsibility of accepting testimony regarding the new moon and declaring the new month. This obligation is a positive mitzvah. The declaration of the new moon is fundamental to establishing the dates of the festivals and – the annual Torah calendar. However, the annual calendar cannot be put in place simply by declaring each new month. The courts must consider another issue. Another passage in our parasha identifies this issue.
“Today you go forth in the month of the springtime.” (Shemot 13:4)
Hashem identifies the month of the redemption from Egypt as the month of the springtime. From this passage, the Sages understood that Pesach must be celebrated in the springtime. However, this requirement creates a dilemma. The seasons are determined by the solar year. In other words, each season occurs at a specific point in the solar year. The lunar year – composed of twelve lunar months – is shorter than the solar year. Pesach occurs on the fifteenth day of the first lunar month. If every year of the Torah calendar were composed of twelve lunar months, it would be impossible for date of Pesach to consistently occur in the springtime. Because a lunar year of twelve lunar months is shorter than a solar year, each year Pesach would occur at an earlier date on the solar calendar. The first Pesach – that was observed in Egypt and the wilderness – occurred in the springtime. However, without some adjustment, in a few years, Pesach would have occurred in the winter! Therefore, the Torah authorized the Sages to occasionally add a thirteenth month to the lunar year. This thirteenth month was used to reconcile the lunar and solar calendars. This reconciliation assures that Pesach always occurs in the springtime. In short, in order to set the annual calendar, the courts must take two steps. First, they must declare the new months. Second, they must occasionally add a thirteenth month to the lunar year – creating a leap year. This additional month reconciles the lunar and solar calendars.
The responsibility of declaring the new month is a positive command. According to Maimonides, the responsibility of the courts to declare an occasional leap year is also included in this commandment. Nachmanides and other disagree. They argue that these two functions are authorized by two separate mitzvot. One mitzvah authorizes the courts to declare the new month and the other authorizes the courts to reconcile the lunar and solar calendars through creating an occasional leap year.
Superficially, Nachmanides position seems to be compelling. The declaring of the new month and the considerations involved in declaring a leap year are two separate functions. How does Maimonides include these two separate functions in one commandment?
It seems that according to Maimonides both of these functions – declaring the new month and creating an occasional leap year – are aspects of one single function. This function is the establishment of the annual calendar. According to Maimonides, there is a single mitzvah. This mitzvah is for the courts to establish the calendar. This single mitzvah includes two elements – declaring new months and creating an occasional leap year.
This explanation of Maimonides’ position has an important implication. According to his position, the courts are charged with the responsibility of establishing the annual calendar. The Torah calendar is lunar. So, the courts must declare each month. But part of the courts obligation in establishing the calendar is to declare an occasional leap year. This implies that the placement of the festivals in their proper season – for example, Pesach in the springtime – is an integral element of the task of establishing the annual calendar. Therefore, the mitzvah of creating the annual calendar requires that the courts consider the seasonal timing of the festivals and evaluate the need for a leap year. Let us express this in simpler terms. The single mitzvah that Maimonides describes can best be defined as an obligation upon the courts to establish the times of observance of the festivals.
Let us now return to the custom of reciting an abridged Hallel on Rosh Chodesh. As Tosefot explain, the Sages enacted a requirement to recite a complete Hallel on festivals. The recitation of this Hallel is part of the observance of the festival. This is directly relevant to the custom of reciting an abridged Hallel on Rosh Chodesh. According to Maimonides, the declaration of Rosh Chodesh is part of a more general mitzvah of establishing the times for observance of the festivals. It is reasonable to assume that in our observance of Rosh Chodesh, we are fulfilling the same mitzvah. By observing each Rosh Chodesh, we acknowledge the new month and participate in the establishment of the annual calendar and the times for the observance of the festivals. This fundamental element of our observance of Rosh Chodesh is expressed through the recitation of an abridged Hallel on these days. The recitation of the abridged Hallel expresses the relationship between our observance of Rosh Chodesh and the observance of the festivals. Through reciting the abridged Hallel we are acknowledging that this observance is directly related to our observance of the festivals.
 Mesechet Ta’anit 28b.  Mesechet Ta’anit 28b.  Mesechet Ta’anit 28b.  Tosefot, Mesechet Ta’anit 28b.  Rav Aharon HaLeyve Epstein, Aruch HaShulchan, Orech Chayim 422:6  Rabbaynu Moshe ben Maimon (Rambam / Maimonides) Mishne Torah, Hilchot Kiddush HaChodesh 4:1.  Rav Aharon HaLeyve, Sefer HaChinuch, Mitzvah 4.