Sfeka d'yoma (Talmudic Aramaic: ספיקא דיומא, lit. "doubt about the day",[1] or doubt regarding the exact date of the day[2]) is a concept and legal principle in Jewish law[3] which explains why some Jewish holidays are celebrated for one day in the Land of Israel but for two days outside the Land.[1][4] The implications of sfeka d'yoma are discussed in Rosh Hashanah 21a and in the commentaries and poskim.[5]