Birkat HaChodesh: Blessing the New Month
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.