Rosh Chodesh which literally means "Head of the Month" is a monthly holiday that celebrates the arrival of the new moon, marking the start of a new month in the Jewish calendar.
The Jewish Calendar is a lunar calendar with twelve lunar months and 354 days per year. Each lunar month consists of 29 1/2 days. Since it is impossible to switch months in the middle of the day, certain Hebrew months have twenty-nine days whilst others have thirty days.
In order for the Jewish Festivals to be synchronized with the proper season, for example Passover (Pesach) which must fall in the spring (in Israel - the Northern Hemisphere), a leap year is added every few years. In a nineteen year cycle there will be seven leap years.
The Jewish leap year, however, is not like the leap year in the Gregorian calendar which is one day added on at the end of February every four years. A Jewish leap year adds on an entire month to close the gap between the 354 day lunar calendar and the 365 day solar calendar. Since the month of Adar (approximately February-March) is the last month of the Hebrew year, we add on an extra month at this time. We then have Adar I and Adar II which assures us that the holidays will be celebrated in the proper season. Originally, the date of Rosh Chodesh was confirmed on the testimony of witnesses observing the new moon, a procedure known as Kiddush Hachodesh (sanctification of the month). After the Sanhedrin (an assembly of either 23 or 71 elders or rabbi's, appointed to sit as a tribunal in every city in ancient Israel) declared Rosh Chodesh for either a full 30 day month or a 29 day month, news of it would then be communicated throughout Israel and the diaspora.
This system was dependent on the functioning of the Sanhedrin to declare the month, and to communicate this month to far-flung Jewish communities. In the 4th century CE, this became impossible and instead a fixed calendar of 29 and 30 day months was instituted by Hillel.
At the end of a 29 day month, Rosh Chodesh is celebrated for one day, on the first day of the new month. At the end of a 30 day month, Rosh Chodesh is celebrated for two days - the 30th day of the previous month, and the 1st day of the new month.
With the exception of the month of Heshvan, the eighth month of the year, every single month of the Jewish Calendar has some sort of special day or special observance. The importance of Rosh Chodesh is therefore obvious.
The names of the months are not Hebrew but Babylonian. The Bible refers to the Jewish holidays only by the order of the months and not by the Babylonian names which were assigned long after the canonization of the Torah. For example, the Torah gives the date of Yom Kippur as the tenth day of the seventh month, whilst the date of Pesach is the fifteenth day of the first month.
The Hebrew month of Av (אָב) is the fifth month of the Jewish calendar when counted from Nissan, and it contains 30 days. It is a month steeped in national sorrow but also contains a spark of deep hope and future redemption. Av is most known for its association with the destruction of both the First and Second Temples in Jerusalem, but Jewish tradition also teaches that the seeds of consolation and renewal are planted in this very time of grief.
The beginning of the month marks the start of the Nine Days, a period of intensified mourning leading up to Tisha B’Av (the 9th of Av). During this time, observant Jews reduce activities that bring pleasure or joy, such as eating meat, drinking wine, listening to music, and bathing for pleasure. This culminates in Tisha B’Av, the saddest day on the Jewish calendar. The Talmud (Ta’anit 26b) teaches that five national tragedies occurred on this day, including the destruction of both Temples. It is observed as a full fast day with restrictions similar to Yom Kippur. We read Megillat Eichah (Lamentations), sit on low stools, and recite kinot (dirges) to remember the devastation of the exile and the loss of our spiritual center.
However, Av does not end in sadness. Just six days after Tisha B’Av, on the 15th of Av, we celebrate Tu B’Av, a day described by the Mishnah (Ta’anit 4:8) as one of the two happiest days of the year (alongside Yom Kippur). In ancient times, young women would dress in white and dance in the vineyards, while young men sought suitable wives. Tu B’Av has become a symbol of renewal, hope, and the power of love—transforming the mourning of Tisha B’Av into joy and unity.
In Jewish mystical tradition, each Hebrew month is associated with a tribe, constellation, Hebrew letter, body part, and precious stone. The tribe corresponding to Av is Shimon, a tribe whose history was marked by intense emotion and zeal. As described in Jacob’s blessings to his sons (Bereishit 49:5–7), Shimon's tendency toward anger was rebuked. Yet, his presence in this month reflects the opportunity for tikkun (repair)—transforming unchecked emotion into disciplined growth. The connection to national mourning and eventual restoration is deeply fitting.
The mazal (zodiac sign) of Av is Arieh (Leo—the Lion). The Midrash Eichah Rabbah (1:28) famously notes the layered lion imagery: "A lion (Babylon) came in the month of the lion (Av) to destroy the Lion of God (Jerusalem), so that a Lion (Mashiach) will come and rebuild it." The lion symbolizes strength, majesty, and divine judgment, but also the noble kingship that will be restored through the lineage of David, the "Lion of Judah." Thus, Av reminds us that even in destruction lies the promise of ultimate redemption.
The precious stone associated with the month—linked through the tribe of Shimon—is sapir (sapphire), one of the twelve stones on the Choshen (High Priest’s Breastplate) (Yoma 73b). Sapphire is a stone of clarity, depth, and spiritual insight. It is said that the Luchot (Tablets of the Ten Commandments) were carved from sapphire (Shemot 24:10), symbolizing the enduring bond between Hashem and Am Yisrael, even when that bond seems strained.
Each month is also associated with a Hebrew letter, and for Av, that is the letter Tet (ט). Tet is the first letter of the word “Tov” (good), which appears for the first time in the Torah in Bereishit 1:4—“And G-d saw the light, that it was good.” Although Tet has a closed, inward-turning form, it reflects hidden good—good that is not always immediately visible. In the month of Av, when we experience national mourning, we are reminded that even pain can carry within it the seeds of future blessing.
According to Sefer Yetzirah, the human limb linked to Av is the left kidney. The Talmud (Berachot 61a) explains that the kidneys offer internal counsel—perhaps representing the inner moral voice. The left side in Kabbalah is associated with din (strict judgment), and indeed, Av begins with severity. But the progression toward Tu B’Av and the comfort of Shabbat Nachamu (the Shabbat following Tisha B’Av) teaches that this judgment is not permanent—it invites introspection, teshuvah, and growth.
The name “Av” itself means father, hinting to Hashem’s role as a loving but guiding parent. Even when He disciplines His children through national trials, it is with the intent of eventual restoration and closeness. Av teaches us that the deepest exile contains the promise of geulah (redemption), and that out of mourning comes new connection.
May we merit to see this month transformed—as the prophet Zechariah (8:19) promises—from a time of fasting into a festival of joy and gladness. May this Av be one of comfort, healing, and spiritual rebuilding—for each of us, and for all of Am Yisrael.