Isru Chag, the day after each Yom Tov, is a term that comes from a Pasuk in Hallel (Tehilim 118), "Kel Hashem Vayer Lanu Isru Chag BaAvosim Ad Karnos Hamizbeiach". What is Isru Chag all about and what does this Pasuk mean?
Rav Chaim Zaitchek (Ohr Chadash - Zos HaBrachah) explains as follows. The Medrash (Vayikra Rabba 31:10) says that dove was allowed in to Gan Eden during the Mabul. Yet this foolish dove left Gan Eden and returned to Olam Hazeh. This is the meaning of Dovid HaMelech's wish "Mi Yitein Li Eiver KaYonah A'ufa V'Eshkona" (Tehilim 55:7). Hashem should give me wings like a dove to fly to Gan Eden, but unlike the dove I will fly there and never leave.
During the glorious period starting with the seriousness of Elul through the awe of Rosh Hashanah, Aseres Yimei Tshuvah, and Yom Kippur, culminating with the Simcha of Succos we are living on a different plane. We live in Gan Eden. During this time of contemplation we are blessed with true insight and inspiration into life and how to live it. We mentally make many decisions about what we want to change, improve, and start anew. But how long does this last, asks Rav Zaitchek? Does it wear off after a few months? weeks? days? or by time Succos is over are we back to business as usual with no real change taking place and all our dreams and fantasies left behind in the Succah?
"Kel Hashem VaYer Lanu", Hashem acts with us with kindness and enlightens us and inspires us for nearly two months. "Isru Chag BaAvosim Ad Karnos HaMizbei'ach". When it's all over we need to bind ourselves and our actions with thick ropes, to the conclusions we've made over the Chag. We need to tie ourselves to the altar on which we've mentally agreed to make so many sacrifices. If we miss this important opportunity we will be saying good bye forever to a lot more than the Succah. We leave the Succah that we dismantled on Isru Chag but are we foolish enough to leave our perch that we've worked so hard to climb?