Faith
Why Is the Thirtieth Day of the Month Also Considered Rosh Chodesh?
Question
How is it that Rosh Chodesh can be two days long, since the first day of the month, meaning the 1st of the month, is only one day? If so, why is the thirtieth day of the previous month also considered Rosh Chodesh?
Thank you
Answer
Greetings,
Our teacher Yeshaya the First zt"l (about 800 years ago) wrote in Shu"t HaRiD (siman 32): Regarding what you asked about Rosh Chodesh, why we make Rosh Chodesh two days, you asked well. Know that the reason for Rosh Chodesh is not like the two days of Yom Tov, for we observe two days of Yom Tov because of doubt, but the two days of Rosh Chodesh are not observed because of doubt, for if so every Rosh Chodesh would be made two days, etc. Rather, the reason is as follows: it is already known that the lunar month is 29 and a half days, and the thirtieth day should have been divided, half to the previous month and half to the next month, but we cannot divide the day into halves. Because of this, the Sages instituted making some months full and some lacking, etc. End of quotation. His words were cited in Shevilei HaLeket (siman 168) and in Birkhei Yosef (siman 427, se'if 1).
With blessing,
Hillel Meirs
