1. It Starts the Jewish Month

Rosh Chodesh” means the “Head of the New [Moon],” and indeed it marks the start of a new lunar month. Thus, Rosh Chodesh is always celebrated right around the time when the new sliver of moon appears in the night sky.

Read: What Is Rosh Chodesh?

2. It Was Once Declared on an Ad-Hoc Basis

In ancient times, Rosh Chodesh was declared by the beit din (Jewish court) after two credible witnesses would testify that they had seen the new moon. Since the fourth century, however, it has been determined by a preset calendar.

Read How This Was Done and Why It Changed

3. It Has One or Two Days

In the now-fixed calendar, with two exceptions, Jewish months alternate between having 29 or 30 days. If a month has 30 days, then day 30 of that month becomes Rosh Chodesh, as does the following day, which is day 1 of the next month. If a month has just 29 days, then the following month has just one day of Rosh Chodesh, day 1 of the month.

Read: 17 Jewish Calendar Facts

4. We Wish People a “Good Month”

On Rosh Chodesh, the proper Hebrew greeting is chodesh tov, a “good month,” or ah gutten chodesh, which means the same thing in Yiddish.

Read: How Jews Do Holiday Greetings

5. There Were Special Sacrifices Brought in the Temple

In the Holy Temple, special, additional sacrifices were brought every Rosh Chodesh: two bulls, one ram and seven lambs. In addition, a single goat was brought as a sin-offering.

Even though the Temple no longer stands, we commemorate these additional sacrifices by praying the Musaf (“Additional”) prayer at the end of morning services on Rosh Chodesh.

Read: Korbanot: The Biblical Temple Sacrifices

6. The Prayers Are Special

In addition to the Musaf, like on Jewish holidays, a special paragraph, Yaaleh Viyavo, is added to the Grace After Meals, as well as to the Amidah (Silent Prayer) on Rosh Chodesh. We also chant Hallel, selected verses from Psalms, in which we praise G‑d for His kindness. Since we omit some paragraphs from the Hallel service, this is commonly known as “half Hallel.”

Read: Why Is Hallel Sometimes Whole and Sometimes Half?

7. Four People Are Called to the Torah on Rosh Chodesh

During morning services, we read Numbers 28, where G‑d dictates the Rosh Chodesh sacrifices, from the Torah. Four aliyahs are given, more than on a regular Monday or Thursday, when just three are given, but less than on holidays, when there are five aliyahs.

Read: Torah Reading for Rosh Chodesh

8. It is Preceded by Shabbat Mevarchim

The Shabbat before Rosh Chodesh is known as Shabbat Mevarchim, “the Shabbat when we bless.” On this day, during the synagogue service, we recite a special blessing for the new month and announce the timing of Rosh Chodesh. In Chabad circles, this Shabbat is also celebrated by reciting the entire Book of Psalms, studying Chassidic texts together and then holding a farbrengen.

Read: 13 Special Shabbats on the Jewish Calendar

9. The Day Before Is Yom Kippur Katan

Reflecting the fact that a sin-offering was brought on Rosh Chodesh, the day before is known as Yom Kippur Katan, Minor Yom Kippur, observed by the most pious with extra prayers, inspiring sermons, fasting and charity.

The Rebbe would customarily visit the Ohel, the resting place of his predecessor, on Yom Kippur Katan.

Read: What to Expect at the Ohel

10. Some Hold Feasts on This Day

Some have the custom of making one of their meals more festive by breaking bread and having other delicacies. Others take it a step further and hold elaborate feasts, replete with Torah teachings and more.

11. It Is Dear to Women

While work is generally permitted on Rosh Chodesh, many women have the custom of abstaining from certain forms of work on this day. This harks back to the episode of the Golden Calf, when the women declined to participate in the "fundraising" effort and refused to surrender their jewelry for use in making the idol. As a reward, they were given Rosh Chodesh as a day which they observe more than the men.

Read: How Does a Jewish Woman Celebrate Rosh Chodesh?

12. Rosh Hashanah Is Technically Also Rosh Chodesh

Rosh Hashanah, the first day of the Jewish year, is also Rosh Chodesh, the first (and second) day of the month of Tishrei. Yet, the Rosh Chodesh elements of the day are largely eclipsed by the Rosh Hashanah observances.

Read: Why No Mention of Rosh Chodesh on Rosh Hashanah?