Yom Tov Shel Rosh Hashana: 5666 (Hebrew: ספר המאמרים תרס״ו‎), or Samech Vov, is a compilation of the Chasidic treatises by Rabbi Sholom Dovber Schneersohn, the fifth Rebbe of Chabad, from the Hebrew year 5666 (1905-06). This series of Chassidic essays are considered a fundamental work of Chabad mysticism. The Samech Vov series is one of the single largest works of Chabad philosophy. The work is titled as Yom Tov Shel Rosh Hashana after the opening words of the first treatise.