Tzedakah on Erev Sukkos:
The Mateh Efraim writes (Siman 625, Ois 21): "One should give a lot of tzedakah on erev Sukkos and invite poor and upright guests for Yomtov. Everyone should do this as much as he can afford."
The K'tzeh Efraim (Siman Koton 25) adds the following:
The Gemara says (Shabbos 133B): "'This is my G-d and I will beautify him.' This means to beautify His mitzvos. Make a beautiful sukkah, a beautiful lulav, a beautiful shofar, beautiful tzitzis, a beautiful sefer Torah, written for His sake with beautiful ink, with a beautiful quill, and with an expert scribe, and tie it with beautiful thread. Abba Shaul says: To beautify Him means to make yourself similar to Him. Just like He is full of grace and mercy, you should be full of grace and mercy."
Rav Yehuda Assad zt"l explains Abba Shaul's words by saying that if someone makes a beautiful sukkah, etc., it may seem to him that he has fulfilled his obligation to glorify Hashem, and he will not give sufficient tzedakah. Thus, he stresses that it isn't enough to beautify mitzvos. One must also emulate Hashem's middos and show compassion to others by giving tzedakah.
Giving Tzedakah to Merit Shelter in His Shade:
The Bnei Yissochor (Tishrei, Maamar 10, Ois 24) quotes Arizal zy"a as saying that one should give a lot of tzedakah on erev Sukkos.
The Chida zt"l (Sefer Kisei Dovid, Drush 7) explains the reason for this by quoting Chazal, who say that when Klal Yisroel left Mitzrayim, they made a pact to do chesed to each other. Mahar"a Meyuchis explains this pact by quoting the Medrash (Rus Rabbah 5:2) that states: "Rav Avin said: Come and see the great power of tzadikim, the great power of tzedakah, and the great power of acts of chesed. They don't take refuge in the shadow of the land... Rather, they take refuge in the shadow of Hashem Himself, as is stated (Tehillim 36:8): 'How precious is Your kindness, Hashem, and the sons of man will take refuge in the shadow of Your wings.'"
He explains this to mean that those who give tzedakah and do chesed are not shielded by any angels or other entities. Rather, they are protected directly by Hashem. Accordingly, the Medrash is saying that when the Jews left Egypt, they made a pact to do chesed to each other so that they should be guided directly by Hashem, and not through any angel or intermediary. And in this merit, they were able to have Hashem walk before them day and night (Shemos 13:21) and protect them at all times.
On Sukkos, we commemorate the Ananei Hakavod, which were a sign of Hashem personally protecting and caring for Klal Yisroel in the merit of them giving tzedakah and doing chesed for each other. Therefore, it is appropriate to approach this day by giving tzedakah and helping our fellow Jews. When we do this, we too will merit being guided directly by Hashem and not through any intermediary. adds that this is the reason why Avrohom Avinu visits us on Sukkos, as he was the first man to truly engage in chesed.
The Sanzer Rov's Tzedakah:
The distribution of tzedakah of the Sanzer Rov zy"a was legendary. Throughout the entire year, he would give away every dollar he had to the poor and needy. He would never go to sleep at night if even one penny remained in his house. But he would give even more tzedakah during the days before Sukkos.
His grandson, Rav Shlomo of Bobov zy"a (the son of his son, Rav Meir zy"a) related that one year on erev Sukkos, the Sanzer Rov said to his sons, "Give me all the money you have. I need it to obtain the most beautiful sukkah decorations in the world."
He explained that on erev Sukkos one must give as much tzedakah as he can because we are visited on Sukkos night by the Ushpizin of Avrohom Avinu, who personified the middah of chesed. We can only properly greet him if we try to emulate him by giving to others.
That erev Sukkos, he distributed 1,800 gold coins to the poor. By noon, he had run out of money. He told someone to go out in the street and ask the esrog dealers to come to his home. He told them, "You sold a lot of esrogim. You certainly have some money in your pockets right now. Please lend it to me, and I'll pay you back when I can. I need the money for my sukkah decorations."
They lent him the money, and he gave that away to the poor as well.
His son, my grandfather Rav Yeshaya of Tchechoiv zy"a, recalled that when he was a boy, he once made a picture of an eagle for a sukkah decoration. When he showed it to his father, the Sanzer Rov told him, "Very nice! Now let me show you my sukkah decoration."
He then showed him how he gave away money to the poor, and he said, "This is my sukkah decoration.”
Lending Sukkah Boards to the Poor:
Rav Mordechai of Lechovitch zy"a was accustomed to preparing many wooden boards so that he could lend them to poor people to use for their Sukkahs.
One year, a poor shoemaker came to him on Erev Sukkos. The man was limping on one leg and said that he needed to borrow some boards to make a Sukkah. The tzadik told him that he had already lent out all of the boards that he had. The poor man then went out to try to find boards elsewhere.
Rav Mordechai watched him from his window and saw that he was going from house to house asking if anyone had extra boards. He felt terrible for the man and cried out, "Ribono Shel Olam. Do you see how much the Jewish people love Your mitzvos? It is raining outside, yet this poor shoemaker who walks with a limp is shlepping form door to door to try to find wooden boards so that he can fulfill Your mitzvah. Please send some boards from Shomayim so that he can have a Sukkah."
Rav Mordechai then went up to the roof of his house and found a few boards laying there. He told his shamash to run and bring them to the shoemaker and to help him build his Sukkah.