2. The source of the name is the verse in Psalms: “Tie the sacrifice (isru chag) with thick ropes to the corners of the altar.” The sages explain “The verse considers one who makes a special meal on this day (thereby “connecting” this day to the previous holiday), as if he built an altar and offered a sacrifice on it.”
3. The Talmud Yerushalmi (Avodah Zara 39) calls Isru Chag “a derivative of the festival.”
4. It is stated about Isru Chag: “Every Jew should connect an extra day to the festival by serving food and drinks and making a special meal, and to add something in his meal so it’s evident that he is having this meal to connect the weekday to the festival. It will be an atonement for him, as if he had offered sacrifices. Fortunate is the person who does it, and makes a meal together with his friends, and says songs and praises of G-d at his table. It is then truly in place of the altar, and is as if he built an altar and offered a sacrifice on it. G-d will consider it like a sin-offering to atone for him in case he sinned during the festival.”
5. Isru Chag is a regular weekday, besides in the following: we have meals that are slightly more festive, we reduce eulogies and avoid fasting, and don’t say Tachanun in the prayers. If Isru Chag falls on as Shabbat, we don’t say Tzidkatcha Tzedek in the afternoon prayer. Besides this, the prayers on this day are the same as on a regular weekday, without Hallel or Mussaf. Tefillin is also worn as during any other weekday.
6. During the pilgrimages that took place on the festivals to the Temple, some of the pilgrims would remain in Jerusalem during the day after the festival. This is why this day is considered connected to some degree to the festival.
7. According to the Aruch HaShulchan (#5), since the festival peace-offerings that were brought on the last day of the festival could be eaten until the end of the following day, many festival sacrifices were also eaten on the day after the festival. The festival joy was therefore connected to the weekdays through the holiness of the sacrifices.
8. The Sdei Chemed (Klalim, Maarechet 1) writes that the reason for Isru Chag in the Land of Israel is to maintain unity with our brothers in the diaspora, who are still celebrating the second festival day which is kept abroad.
9. Our sources say: “Whoever maintains his joy on Isru Chag, has fulfilled the words of our sages and will receive reward for it.”
10. Our sources also say that a person who is happy on Isru Chag, demonstrates that he loves G-d’s commandments and finds it difficult to take leave from them. “He also testifies about himself that he loves the holiday which the Torah commanded him to keep, and finds it difficult to suddenly leave it. He wants to hold on to it as much as he can even when it is over. This is how holy Jews feel. They love the commandments and can’t wait until they can keep them, and do their utmost to hold on to them even when their time is over.”