Birkat HaChodesh: Blessing the New Month
Question
The prayer of Birkat HaChodesh includes an announcement of when Rosh Chodesh will be. But, originally, the determination of which day Rosh Chodesh would fall out on would depend on the court’s declaration. Even though Chazal knew when Rosh Chodesh should fall out, there was no guarantee that there would be witnesses to allow for the declaration to be made on that specific day. It therefore seems that the announcement of when Rosh Chodesh will fall out in the upcoming week is inappropriate.
Answers
You are absolutely correct. In the era that witnesses would appear and the Sanhedrin would declare the new month, there was no Birkat HaChodesh. Birkat HaChodesh was added into the Shabbat services later on, after the destruction of the Holy Temple, to let people know that there was going to be a new month.
Why is Birkat HaChodesh recited on Shabbat? In the past, many people were not able to get to Shul during the week, and subsequently they may not have been aware that Rosh Chodesh would be that week. However, all people usually made it to Shul on Shabbat. Therefore, the Rabbis instituted making an announcement to let everyone know that Rosh Chodesh was imminent. Although it is now universally said on Shabbat day, several hundred years ago there were communities where the announcement was made in Shul on Friday night.
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