The day immediately following the three pilgrimage holidays—Passover, Shavuot and Sukkot—is called Isru Chag (אסרו חג)‎, which literally means "bind [the] festival" since the day is “bound” to the holiday.

The name is based on the verse in Psalms, “Bind [the] festival [offering] with cords to the corners of the altar.”1 The word translated as “cords” can also mean “fattened animals.” Thus, the Talmud understands that “anyone who establishes an addition [isur] to the festival on the day after the festival by eating and drinking is credited by Scripture as if he built an altar and sacrificed an offering upon it.”2

Appropriately, we read in the Code of Jewish Law that on Isru Chag we add something festive to the meal of the day and omit Tachanun (confessional supplications) from our prayers.3

What is the reason for this celebration?

Finish Eating the Offerings

Some trace this back to the Temple offerings.4 When a person brought the required festival shelamim (“peace”) offering, he would be allowed to partake of its meat on the following day as well. Thus, if a person brought the offering on the first day of the holiday, he could eat it on the second day. It follows that if one brought it on the last day, he could enjoy the shelamim meat on the day after the holiday as well.

In this sense, the day after Shavuot is even more festive than the days following Passover and Sukkot. Passover and Sukkot extend over an entire week, so there was time for all offerings to be brought during the intermediate days of the holiday. However, since Shavuot is just one day (or two in the diaspora), most of the sacrifices were brought the day after Shavuot, which became known as Yom Tavoach, the “day of slaughter.” (In the event that Shavuot coincided with Shabbat, the festival peace offering could only be brought on this day.)5 This is why fasting is actually forbidden on Isru Chag Shavuot (and only avoided due to tradition after the other holidays).6

Bringing the Holiness into the Mundane

The Chassidic masters point out that on the one hand, Isru Chag is “bound” to the holiday, celebrated with special festivities. On the other hand, it is a regular weekday. Thus, as the afterglow of the holiday, it bridges the holy and the mundane, drawing down the spiritual energy and blessings of the holiday into our everyday lives.7