Tractate Rosh Hashanah, located in Seder Moed (“The Order of Festivals”) opens with the statement that there are four days of the calendar, each of which is a New Year for its own purpose. The bulk of the tractate's discussion is elaboration of the laws concerning the holiday of Rosh Hashanah, its religious significance, and the details of the sounding of the shofar. The tractate also discusses the process of sanctifying a new month in the Temple period, which involved witnesses testifying in front of the high rabbinic court that they saw the new moon.
