In the coming year, 5784, we will not fulfill the mitzvah of Tekias Shofar Min Hatorah, and the same for the Mitzvah of Daled Minim. Min Hatorah, we are obligated to blow the shofar on the first day of Rosh Hashanah, and perform the mitzvah of Daled Minim on the first day of Sukkos. This year, with the first day of both falling on Shabbos, we will not blow Shofar nor shake Lulav on those days.
The reason we do not perform these Mitzvos on Shabbos is due to a Rabbinic decree enacted because of concerns regarding Chilul Shabbos. Chazal were given the power to enact Takanos and Gezeiros in order to ensure that the laws of the Torah are upheld, and we are Biblically commanded to listen to our Chachomim.
The question then arises - if Chazal decree that a Mitzvah should not be performed, and one disregards that decree and performs the Mitzvah, has he actually fulfilled the Mitzvah? Does Chazal's decree simply absolve one of the obligation to perform the Mitzvah or does it completely "cancel" the Mitzvah? For example, if one blows Shofar this year on the first day of Rosh Hashana (that falls on Shabbos), he violates the Rabbinic decree - but has he fulfilled the Mitzvah of Shofar?
This question has been discussed by our Rishonim and debated by our Acharonim throughout the generations, and has many ramifications in a number of Mitzvos that are illustrated in this booklet.