756. Isru Chag and Chol HaMoed

103:14 It is customary on the day after each of the three Festivals to slightly increase what one eats and drinks. This day is called “isru chag.” The practice is not to fast on isru chag – not even a bride and groom on their wedding day, nor on a yahrzeit, as some otherwise do. (According to 146:2, a bride and groom don’t fast on the isru chag of Shavuos, but it would seem that they do on other days of isru chag. See also Mishnah Brurah 494:6.) On the isru chag that follows Shavuos, it is outright forbidden to fast because, while the Temple was standing, if Shavuos fell on Shabbos, sacrifices were slaughtered the next day; those of Pesach and Succos were brought on the first day of chol hamoed.

104:1 On the intermediate days of a yom tov – “chol hamoed” – some forms of labor are permitted and others are prohibited. Whatever is necessary for preparing food for chol hamoed or yom tov is permitted, as is work required to save a person from incurring a financial loss. One must be very careful not to perform forbidden forms of labor on chol hamoed since our Sages said (Pesachim 118a) that one who profanes chol hamoed is like one who worships idols.”