Introduction to Rosh Chodesh
Rosh Chodesh, the beginning of the Jewish (lunar) month, is a minor holiday of one or two days, depending on whether the previous month was 29 or 30 days. When the preceding month is 30 days long, the 30th day becomes the first day of Rosh Chodesh and the second day of Rosh Chodesh is the first day of the succeeding month.
Rosh Chodesh used to be officially declared in Jerusalem each month by the Sanhedrin, based on evidence from at least two witnesses who had seen the new moon in the western sky. For the past 1,700 years or so, the new moon (and the dates for the entire Jewish calendar) have been determined by a formula prescribed by Hillel HaNasi (then head of the Jewish community in Eretz Yisrael).
The molad (first appearance of the new crescent moon somewhere in the world but using Jerusalem time) is an average molad and may be more than 12 hours off the actual time of the moon’s first appearance. It varies somewhat from month to month.
Ya’aleh V’Yavo on Rosh Chodesh
If you forgot (or are not sure if you said) ya’aleh v’yavo of:
Rosh Chodesh Ma’ariv
Don’t repeat the amida. This applies to both ma’arivs on a two-day Rosh Chodesh.
Rosh Chodesh Shacharit or Mincha
If you forgot: Repeat the amida of Rosh Chodesh shacharit or mincha
If you are not sure: Repeat the amida with the condition that if you had said ya’aleh v’yavo the first time, the second time is a voluntary prayer (tefilat nedava).
NOTE: If Rosh Chodesh falls on Shabbat and you are not certain whether you said ya’aleh v’yavo at shacharit or mincha, you must repeat the amida without a condition.
Shabbat-Rosh Chodesh: Adding Ul’Chaparat Pasha
SITUATION: It is Rosh Chodesh in a Jewish leap year.
WHAT TO DO: Add “ul’chaparat pasha” to musaf—from Rosh Chodesh Cheshvan through and including the second month of Adar.
REASON: This blessing usually contains 12 requests–corresponding to the 12 months–and so in a leap year, we add ul’chaparat pasha for the 13th month.
NOTE: Don’t say ul’chaparat pasha on Rosh Chodesh Nisan or after that until the next Jewish leap year.