Zmanim · The calendar and the moon
What is Rosh Chodesh?
Rosh Chodesh, literally the head of the month, is the beginning of each Hebrew month, arriving with the new moon. It is a minor festive day: it is announced in advance on the preceding Shabbat, and it is marked in prayer with Hallel and with the added passage Yaaleh v'yavo. Depending on the length of the outgoing month, it lasts one day or two.
The turn of the month
The Jewish month is a moon, and Rosh Chodesh is the moment it begins again. It is not one of the great festivals, yet the tradition does not let it pass unmarked. There is a quiet dignity to it: a monthly reminder that time renews, that the calendar is not a flat line but a series of fresh beginnings written into the sky. When the outgoing month has thirty days, Rosh Chodesh is kept for two days, the last day of the old month and the first of the new; when it has twenty-nine, it is a single day.
How it is announced and how it is prayed
Rosh Chodesh casts its shadow forward. On the Shabbat before it, שבת מברכים (Shabbat Mevarchim, the Shabbat of blessing), the coming month is announced in the synagogue so the whole community knows when it falls. On the day itself, the prayers change: Hallel, the psalms of praise, is added, and the passage יעלה ויבא (Yaaleh v'yavo, 'may it rise and come'), which asks Hashem to remember His people on this day, is inserted into the Amidah and the grace after meals.
The source
The day itself is rooted in the Torah, in Numbers 28:11, which sets out the offering brought at the beginning of each month. The way the new month was established, through witnesses who came to testify that they had seen the new moon, is discussed in the Talmud, Rosh Hashanah 20a, where the court's work of fixing the month is weighed with great care.
What to do
Know when Rosh Chodesh falls this month, and whether it is one day or two. When it comes, add Hallel where your community says it and remember Yaaleh v'yavo in the Amidah and in the blessing after eating. And on the Shabbat before, listen for the announcement of the new month; that is the tradition handing you the coming weeks in advance.
Source: Numbers 28:11; Talmud, Rosh Hashanah 20a.
So Rosh Chodesh never slips past you, Kimá (the Jewish clock: your city's zmanim and candle times) marks the new month and its days for your place, turning the moon's renewal into a date you can actually keep.
Open Kimá