On Ereb Succot, the Misva of the day is to dispense Sedaka to the poor. The custom of the Arizal (Rav Yishak Luria of Safed, 1534-1572) was to separate the coins for the Pidyon Nefesh on Ereb Rosh Hashana, perform the Pidyon on Ereb Yom Kippur and distribute the money to needy Talmideh Hachamim on Ereb Succot. It should be noted that the Sedaka should be distributed in honor of the Ushpizin (the seven supernal guests). This is based on the teaching of the Zohar that the seven Ushpizin only come to a person’s Succah if he also hosts poor people at his table. The commentaries explain that this also includes providing for the holiday needs of the needy before Hag. If a person didn’t give charity before Succot, the Ushpizin don’t want to come to the Succah of a selfish person.
The Gaon of Vilna (Rav Eliyahu of Vilna, 1720-1797) revealed a Remez (an allusion) to this practice in the Torah. The Succah is a commemoration of the "Ananeh HaKavod" (Clouds of Glory) which surrounded and protected the Jewish people when they left Egypt and travelled in the desert. The question is why we celebrate Succot in Tishri, when the miracle actually occurred upon leaving Egypt in Nisan. The Gaon explains that these clouds were taken away when the Jews sinned with the Golden Calf. Without them, they became exposed and vulnerable to enemies and the elements. Even after Moshe Rabbenu achieved forgiveness for the Jews on the tenth day of Tishri, Yom Kippur, the clouds did not return. After Yom Kippur, Moshe instructed the Jews to donate for the building of the Mishkan as atonement for their sin. During those three days, the Jews donated enthusiastically until on the fourth day, the fourteenth of Tishri, Ereb Succot, Moshe told them that they had already contributed more than enough, and everything was in place to begin building. In the merit of their wholehearted participation in the campaign, Hashem fully forgave them and returned the clouds of glory on the fifteenth of Tishri, the holiday of Succot. Thus, we celebrate Succot, not in Nisan to commemorate the original clouds, but in Tishri, on the very day that the Clouds of glory were returned, signifying Hashem’s love and the restoration of our previous grandeur after the sin. This is also the allusion to the Misva of giving large sums of Sedaka on Ereb Succot, echoing the generosity shown by the Jews in donating to building the Mishkan.
This idea that the Succah symbolizes the restoration of Hashem’s original endearment is also alluded to in the Yom Tob prayer "Ata Vehartanu." "Ata Vehartanu MiKol Ha’amim" (You chose us from all the nations) refers to the Jews’ status when leaving Egypt. "Ahabta Otanu" (You loved us) refers to our relationship when receiving Torah. "Verasita Banu" (You desired us) refers to Hashem’s renewed love after rectifying the sin of the Golden Calf.
On Succot Hashem invites us once again to enter his Clouds of Glory, in the merit of the Teshuba of Yom Kippur and the Sedaka before Succot.
It is an important Misva to distribute Sedaka to needy Talmideh Hachamim on Ereb Succot. This should be done in honor of the Ushpizin.