It is a good custom to eat and drink a little more than usual on the day following a festival (Rema 429:2), since the aura of the festival spills over into it. In the Yerushalmi, this day is referred to as Ben Ha-mo’ed (y. AZ 1:1), whereas in the Bavli, it is referred to as Isru Ḥag, meaning the day that is tied to the festival. By adding a little joy to Isru Ḥag, we demonstrate that the festival is dear to us and that it is difficult for us to let it go.
The Sages state: “Whoever makes an addition to the festival by eating and drinking is regarded by Scripture as though he had built an altar and offered a sacrifice on it. For it is said (Tehilim 118:27): ‘Make an addition to the festival (isru ḥag) with fat cattle, up to the horns of the altar’” (Sukka 45b). What does this mean? When a person eats for the sake of heaven so that he will have the strength to perform good deeds, and he invites guests in order to make them happy, and he speaks words of Torah at the table, then “a person’s table atones for him” (Berakhot 55a; Menaḥot 97a; Tosafot and Maharsha ad loc.; Avot 3:3). The foundation for the holiness of the table begins with the festivals, when there is a mitzva to prepare festive meals. Accordingly, if one adds a festive meal to the day following the festival, he is extending the festival’s holiness and values into the rest of the year. This is why it is as if he has built an altar and offered a sacrifice to God.
Since the aura of the festival spills over somewhat into Isru Ḥag, the custom is not to eulogize or fast then. Nevertheless, according to the letter of the law, it is not forbidden to do so (SAH 429:17; Kaf Ha-ḥayim 494:48).
In contrast, on Isru Ḥag of Shavu’ot, even the letter of the law prohibits eulogizing and fasting, because it is “a day of slaughter,” meaning a day when sacrifices are brought. Sometimes the festival offerings were sacrificed then. For example, if Shavu’ot was on Shabbat, the olat re’iya and shalmei ḥagiga would be postponed until Sunday, which was Isru Ḥag. Other years as well, many people did not manage to offer all their sacrifices on the festival, so the following day they offered what remained. A day on which sacrifices are offered is considered a day of simḥa. Therefore, one may not eulogize or fast then (SA 494:3; Levush; MA ad loc. 3; SAH ad loc. 19).