Contributed by: R. Ezra Mizrahi
Rosh Hodesh Tammouz always consists of two days, for the preceding month of Sivan always has thirty days - thus, the thirtieth day of Sivan is the first day of Rosh Hodesh Tammouz. This is the rule whenever a preceding month has thirty days: the following month will have two days of Rosh Hodesh, the first being the thirtieth day of the prior month, and the second day of Rosh Hodesh being the first day of the new month.
The month of Tammouz always has twenty-nine days. Therefore, the month of Av, which follows Tammouz, always has only one day of Rosh Hodesh.