Rosh Chodesh: 10 Things You Might Not Know

The Jewish month follows the moon’s cycle, which takes nearly 30 days. At the start of each month, the moon looks like a thin sliver. It becomes full halfway through and disappears completely at the end. When it appears again, this moment is called the molad, the "birth of the moon." That’s when Rosh Chodesh begins. Since the moon’s cycle is slightly longer than 29½ days, some months are 29 days (called “deficient”), while others are 30 (called “full”).

Even though Jewish scholars have long known how to calculate the start of each month, the Torah tells us to sanctify the month only after seeing the new moon. When a Jewish court (beit din) still had full authority in Israel, two witnesses who saw the new moon would come and testify. If their testimony was accepted on the 30th day, that day became Rosh Chodesh, and special sacrifices were brought. If no one came, the next day, day 31, was automatically set as the start of the new month.

Today, we no longer rely on live witnesses. Our calendar is fixed according to a system set by Hillel II almost 1,700 years ago, when he realized Jewish leadership in Israel was about to end. Thanks to his calendar, we know in advance when Rosh Chodesh will fall.

Ever wonder why there are two days of Yom Tov (holiday) in the Diaspora? It’s actually related to the old way of determining Rosh Chodesh. Long ago, messengers were sent from Israel to let Jewish communities know when the new month had been declared. But if they didn’t arrive in time, people outside Israel didn’t know exactly when holidays began. So, they kept two days to be sure. That tradition continues today.

Rosh Chodesh is meant to be celebrated. It’s a mitzvah to have a festive meal, even a poor person should try to enjoy something special, like a favorite fruit.

After the morning prayer (Shacharit) on Rosh Chodesh, we say Hallel with some parts left out. We also read from the Torah, add Yaaleh V’Yavo in the Amidah and Birkat Hamazon (Grace After Meals), and say the special Musaf prayer for Rosh Chodesh.

Rosh Chodesh is a special day for women. Because they didn’t take part in the sin of the Golden Calf, they were rewarded with this holiday. Many women have the custom to avoid hard work on Rosh Chodesh.

On the Shabbat before Rosh Chodesh, we announce the coming new month in synagogue. That Shabbat is called Shabbat Mevarchim, the “Shabbat of Blessing,” because we bless the upcoming month.

Rosh Chodesh plays a big role in keeping the Jewish calendar and holidays accurate. That’s why some oppressive rulers tried to ban it. Kings like Antiochus Epiphanes and Hadrian tried to stop Jews from sanctifying the new month but they didn’t succeed.

In the times of the First and Second Temples, Rosh Chodesh was a bigger celebration than it is today. People would stop working, bring special sacrifices, share festive meals, and gather to hear words of Torah from their sages.