Is it correct that the first day of a two day Rosh Chodesh (the beginning of a new month) is called Rosh Chodesh of the next month’s name, even though it’s the 30th day of the previous month? For instance, the month of Sivan has 30 days, and the following month is Tammuz. Is the 30th day of Sivan called Rosh Chodesh Tammuz?
Yes, whenever there are two days of Rosh Chodesh they are both named Rosh Chodesh of the second month.
The reason that some months have two days of Rosh Chodesh and some have only one depends on whether the month has twenty nine or thirty days. If it is thirty days the thirtieth day will contain the actual "birth" of the new moon and it is considered to be the first day of Rosh Chodesh and the first day of the new month is the second day of Rosh Chodesh. When the month is only twenty nine days long, the new moon is "born" on the next day, the first day of the new month, which is the only day of Rosh Chodesh.
All the months with the exceptions of Cheshvan and Kislev have the same number of days every year.
Tishrei - 30 days
Cheshvan - 29 or 30 days
Kislev - 29 or 30 days
Tevet - 29 days
Shvat - 30 days
Adar - 29 days (when there are two months of Adar the first one always has 30 days)
Nisan - 30 days
Iyar - 29 days
Sivan - 30 days
Tamuz - 29 days
