Sectarianism in the Jewish religion increased during the Second Temple period. One of the prominent manifestations of their differences was the establishment of a different religious calendar for each sect, causing a ‘war of the calendars.’ The first part of this book provides a comprehensive survey of the various calendars found in the ancient world. The second part discusses the redaction of the Book of Psalms in this light. It argues that the redactional Book of Psalms is according to the lunar calendar, while that of the “Psalms Scroll” (11QPs) is according to the solar calendar as practiced at Qumran. Variations in the redaction of the Masoretic, the Septuagint and the other five identified Psalter Scrolls found in Qumran are interpreted as corresponding with the variations in the lunar and solar calendars.