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.
Nissan marks the first month of the Jewish year, as described in Exodus 12:2, where the Almighty tells Moshe, "This month shall be for you the first of the months." This month spans 30 days and is one of the few mentioned by name in the Torah.
Nissan is referred to as Chodesh HaAviv—the Month of Spring—which is significant in maintaining the delicate balance of the Jewish calendar, including leap years.
Nissan holds profound historical importance as it is the month when the Israelites were freed from slavery in Egypt. The Passover (Pesach) festival commemorates this momentous event, celebrated annually on the 15th of Nissan, marking the anniversary of the Exodus.
Nearly a year after their liberation, the Israelites inaugurated the Tabernacle—the portable sanctuary built for the Almighty—on the first of Nissan. Over the next twelve days, each of the twelve princes of Israel presented offerings, and we honor this tradition by reading a special Yehi Ratzon prayer and the Torah portion that details the gifts brought each day.
Throughout Nissan, we refrain from saying Tachanun, the confession of sins, out of respect for the joyous nature of the month. The first twelve days, which commemorate the offerings of the princes, are particularly festive, and we also refrain during Passover, continuing this practice for the entire month.
The astrological sign for Nissan is Aries, but in Kabbalistic tradition, the corresponding symbol is a taleh (lamb), representing unity. Lambs are gentle, calm creatures whose voices elicit mercy, much like how the Jewish people, like sheep following a shepherd, accepted the leadership of Moshe. This symbolism also reflects the Jewish people's strength in unity, even in the face of challenges.
The tribe of Yehuda is associated with Nissan. Yehuda, whose name means "praise," is a fitting symbol for the month in which the Jewish people praise the Almighty for our liberation. Yehuda is also the ancestor of King David, from whose lineage the future Messiah is destined to come.
Nissan is tied to the sense of speech, with the belief that the world was created through God's words in this month. The right foot is the body part associated with Nissan, signifying movement and action. The letter/number corresponding to Nissan is hei (ה), representing the number five. Additionally, the stones linked to this month are the Dark Red Garnet and Blue Topaz, each reflecting the vibrancy and significance of Nissan.