Each of the Shalosh Regalim has its own special quality, which illuminates and influences all the days of the year. This is why they are called "Regalim" (literally, "legs"), as they are the foundations upon which the entire year rests. Each festival has its unique influence that pertains to it: Pesach is the "Time of our Freedom," Shavuos is the "Time of the Giving of our Torah," and Succos is the "Time of our Rejoicing."
The Rambam, expounds in Hilchos Succah (8:15) on the great importance of the mitzvah of joy during the "Time of our Rejoicing." His golden words are as follows: "The joy a person feels when performing a mitzvah and in the love of Hashem Who commanded them, is a great avodah. And anyone who refrains from this joy is worthy of retribution..., and anyone who humbles himself and lowers himself in these places is truly great, honored, and serving out of love. So too did Dovid, King of Israel, said (Shmuel II 6:22), ונקלותי עוד מזאת והייתי שפל בעיני' - 'And I will be even more humble than this, and I will be low in my own eyes.' There is no greatness or honor except in rejoicing before Hashem, as it says (Shmuel II 6:14), 'והמלך דוד מפזז ומכרכר לפני ה'' - 'And King Dovid danced and whirled before the Hashem.'"
The Holy Arizal, (in his introduction to Shaar HaMitzvos), wrote that the joy in performing a mitzvah is the essence and root of all mitzvos, as it is written (Devorim 28:47), 'תחת אשר לא עבדת את ה' אלקיך בשמחה' - ‘Because you did not serve the Lord your God with joy.’ And in the sefer Avodas HaKodesh (11:161), Rabbeinu Chida, wrote that the main reward for a mitzvah is for the joy with which it is performed, more than for the actual fulfillment of the mitzvah itself.
It is important to know (as the Vilna Gaon also explains) that the mitzvah of joy is one of the most challenging tasks of the festival. It is forbidden to be sad at all during these days of joy, even for a moment. Just as chametz is prohibited in even the smallest amount during Pesach, so too sadness is forbidden in even the smallest measure during Succos. These are matters of the heart, and it is very difficult to be cautious about them. Therefore, one must make a great effort to be joyous throughout all the days of the festival, and every minute of joy is a mitzvah unto itself!
- Tiv HaMo'adim – Tishrei - Sukkot
