Isru Chag (Hebrew: אסרו חג, lit. "Bind the Festival") refers to the day after each of the three pilgrimage festivals in Judaism: Pesach, Shavuot and Sukkot. The phrase originates from the verse in Psalms 118:27, which states, “Bind the festival offering with cords to the corners of the altar.” This verse, according to the Sages of the Talmud, should homiletically be understood to mean “Whosoever makes an addition to the Festival by eating and drinking is regarded by Scripture as though he had built an altar and offered thereon a sacrifice.”
In a responsum to a community that had inquired as to the rationale behind the observance of Isru Chag, Ben Ish Chai cited Rabbi Isaac Luria to the effect that we connect the day after the holiday to the holiday itself due to the remaining “light” of the holiday – in other words, so that the sanctity of the holiday will be extended.
Read more about Isru Chag: Observances