With this reasoning in mind, how can we begin to count the omer on the second day of Pesach? Counting the omer recalls cutting the omer and bringing it to the Beit Hamikdash – clearly something that can only be done on a weekday. Aren’t we, therefore, demeaning the second day of Pesach by counting on it?
The Kesef Mishneh offers two answers: First, on Shemini Atzeret, the place where one would, in theory, say leishev basukkah is at the end of Kiddush. But in that very same Kiddush, we refer to the day as “Yom Shemini Atzeret hachag hazeh.” If it is Shemini Atzeret, it is not Sukkot and there is no mitzvah to eat in the sukkah. Hence, we can’t say leishev basukkah. We would be contradicting ourselves – declaring the day to be both Sukkot and Shemini Atzeret in the every same Kiddush.
We have no such problem concerning counting the omer. We make a separate blessing on it – unconnected to any other beracha that declares the day to be Pesach.
The Kesef Mishneh offers another very simple answer: Since we now exactly what days the holidays falls on – since we have a fixed calendar – there is no halachic obligation to sit in the sukkah on the eighth day of “Sukkot,” which we know for certain is not Sukkot, but Shemini Atzeret. There is no longer a real sefeika deyoma. Therefore, we don’t say leishev basukkah. For the very same reason, we do count omer on the second day of Pesach since the halacha is clear that we must commence the count on the 16th of Nissan.
(To be continued)