Start of month of Av on the Hebrew calendar. Av (אָב) is the 5th month of the Hebrew year, has 30 days, and corresponds to July or August on the Gregorian calendar. רֹאשׁ חוֹדֶשׁ, transliterated Rosh Chodesh or Rosh Hodesh, is a minor holiday that occurs at the beginning of every month in the Hebrew calendar. It is marked by the birth of a new moon.