isru chag

The day after a yom tov is known as isru chag אסרו חג. Where does the name come from?

Many of us are probably aware that the phrase occurs in Hallel. The verse (Tehillim 118:27) says:

אִסְרוּ-חַג בַּעֲבֹתִים--עַד קַרְנוֹת, הַמִּזְבֵּחַ.

which can be translated as, "Bind (isru) the festival offering (chag) to the horns of the altar with cords" (and remember we saw earlier that the word chag can refer to the sacrifice brought on a holiday.)

The verse is a request from those visiting the Temple to the kohanim.

The gemara in Sukkah (starting on 45a) brings a number of drashot on this verse. The first is a halachic one, that describes how the sacrifice should be prepared on the altar. The second drasha says that anyone who binds the lulav with the hadas is as if he built an altar and brought a sacrifice.

The third drasha says:
כל העושה איסור לחג באכילה ושתיה
"anyone who makes an isur for the chag with food and drink" is as if he built an altar and brought a sacrifice.

What does it mean to "make an isur with food and drink"? Jastrow translates it as "he who creates a circle [makes a band, related to "binds" - isru] for the festival with eating and drinking, i.e. social pleasures." This is the first of Rashi's two explanations. Rashi's second explanation is that "there are those that say 'the day after the chag'." In regard to this explanation, Jastrow writes "Others explain: he who makes an addition to the number of festive days."

The Rama (Orach Chaim 429:2) quotes this second opinion in Rashi, and from here we have the custom to eat and drink a bit more on the day after a holiday.