Lulav on Shabbos

The mitzvah of Lulav originally applied for seven days in the Temple, but for only one day elsewhere. Originally, if the first day was Shabbos, the mitzvah was observed even outside the Temple. After the Temple was destroyed, it was decided that the mitzvah should be observed for seven days everywhere, except on Shabbos, and should not be observed on Shabbos even on the first day. The calendar was arranged to prevent Yom Kippur from falling on (Friday or) Sunday. This means that the first days of Rosh Ha-Shanah and Sukkos can’t fall on Friday. If the calendar had also been set up to prevent the first day of Sukkos from falling on Shabbos, a two-day postponement of Rosh Chodesh would sometimes have been required.