The day after vacation is often a time of distraction and disorientation. The same is true of the day following a religious holiday, especially after one of the week-long holidays (Passover and Sukkot) during which one focuses for an entire week on spiritual, rather than mundane, matters.
In recognition of the challenge of transitioning from a religious festival to everyday life, a semi-holiday known as Isru Chag follows each of the three pilgrimage festivals (Passover, Shavuot and Sukkot.) Translated literally as "bind the festival," the term Isru Chag comes from Psalms 118:27, which reads "Bind the festival offering with boughs to the corners of the altar." From this verse, the sages determined that, "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" (Talmud Sukkah 45b).
In truth, the celebration of Isru Chag has little effect on the day-to-day conduct of most people...unless one is a parent of a child in a religious school (which may be closed for Isru Chag). Isru Chag also affects some aspects of the daily prayer service, in that tachanun (a supplicatory prayer) as well as memorial prayers are omitted, and private fasts are generally not permitted. (Example of a private fast: an Ashkenazi couple who is to wed on Isru Chag will not observe the custom of fasting before the chuppah).
The idea of Isru Chag is that one draws some of the holiness of the festival celebration into the less spiritually elevated reality of everyday life. Since feasting is one of the ways in which Jews celebrate festivals, it became customary to eat and drink a little something extra on Isru Chag to continue the feeling of celebration.
This Treat is reposted in honor of Isru Chag Sukkot.