Question:
Why is 30th of month considered 1st day of rosh chodesh? If because outside of Jerusalem the exact date of rosh chodesh was not known then why is 30th of month considered, in Jerusalem, the 1st day of rosh chodesh? People in Jerusalem knew exactly which day was sanctified as 1st day of month
Answer:
Hello,
The 30th day after the new moon is always observed as Rosh Chodesh. The reason being that the Bais Din would only certify the new moon during the daytime, and it is very possible that witnesses will come the next morning, and the new month will be declared in the morning or afternoon. If that happens then Rosh Chodesh will have started from the night before. Therefore, even in Jerusalem they observed it, as they also didn’t yet know what it would be. By the way this is why even Jerusalen has two days of Rosh Hashana, because it is possible that the witnesses will come during the daytime, and the holiday will technically have started form the night before.
Best wishes
Sources:
See Bais Yosef Orach Chaim 501