ISRU ḤAG (Heb. אִסְרוּ חַג), designation for the day following the three *pilgrim festivals. The name is derived from Psalms 118:27: "Bind the sacrifice (isru hag) with cords, even unto the horns of the altar," which the Talmud (Suk. 45b) interprets: "He who makes an addition (issur) to the festival (ḥag) is considered to have built an altar and sacrificed on it." Rashi comments that this is understood by some to refer to the day after a festival. In the Jerusalem Talmud the day is known as bereih de-mo'ada ("the son of the festival"; tj, Av. Zar. 1:1, 39b). Liturgically, it has been the custom to treat isru ḥag as a sort of minor holiday; no supplicatory and penitential prayers are said, and fasting and funeral eulogies are prohibited.
Eisenstein, Dinim, s.v.
"Isru Ḥag." Encyclopaedia Judaica. . Encyclopedia.com. (November 22, 2018). https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/isru-hag
"Isru Ḥag." Encyclopaedia Judaica. . Retrieved November 22, 2018 from Encyclopedia.com: https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/isru-hag