So today, in halachic or Jewish terms, it's called Isru Chag. The day after a holiday is always called Isru Chag, regardless of whether it's Pesach, Shavuot, or Sukkot. Where does it come from? It's a pasuk (verse) that we say in Tehillim: Isru Chag ba'avotim ad karnot ha-mizbeach (altar). We said it throughout the holiday during the Hallel: Isru Chag ba'avotim. Le'esor, in this case, doesn't mean to make it asur (to make it prohibited), but le'esor is to bind. Isru is bind; Chag is the festival; ba'avotim is offering with cords; ad karnot ha-mizbeach is until the horns of the mizbeach.
The Gemara in Masechet Sukkah says that somebody binds the festival by extending the joy and honor of the Yom Tov (holiday) into the chol (weekday). It's as if he's bringing a korban (offering) on the mizbeach (altar), and that's the importance of this day. The Chachamim (sages), especially the Ben Ish Chai, explain the Kedushah (holiness) all the way from the Yamim Noraim (Days of Awe) until the end of Sukkot, Shemini Atzeret, and Simchat Torah: How do you go from that to a regular day? It's a little bit like—you want to take the Kedushah of those days and bring it into your life. For that, there's a bridge. The bridge is today. Today is the bridge, and that's what the pasuk says. It's a bridge to bring another korban because you're still connected to those days.
And therefore, how does it manifest itself? According to halacha (Jewish law), no eulogy is allowed to be said today. Nobody is allowed to fast today. Again, when I say today, I mean every time a holiday finishes.
And it's a festive spirit. Let's call it half a Chag, half a holiday, in the sense that you could have one more day of good food. You start your diet tonight, not today. Tonight you start the diet, or a semi-diet, but it's a festive day. And therefore, a person wants to keep this—the Ben Ish Chai is very strong on this. Somebody who wants to keep the spirit of the holiday has to have this day as a bridge. And therefore, it's a very important day to be festive and happy on Isru Chag, Bechavod.