Why is the day after yom tov referred to as isru chag? I assume it has something to do with the possuk in tehillim 118:27, where this phrase appears, but I know no further. Any ideas? The earlier the source the better. (It would also be nice to know the earliest source for calling it this, but that might be two questions in one.)
It's based on the Gemara Sukkah top of 45b:
א"ר ירמיה משום ר"ש בן יוחי ור' יוחנן משום ר"ש המחוזי משום ר' יוחנן המכותי כל העושה איסור לחג באכילה ושתיה מעלה עליו הכתוב כאילו בנה מזבח והקריב עליו קרבן שנא' אסרו חג בעבותים עד קרנות המזבח
anyone who makes a 'binding'/'issur' to the Festival with eating and drinking, the verse credits him as though he built an altar and sacrificed an offering upon it, as it is stated: “Bind/'Isru' to the Festival with fattened animals until the horns of the altar.”
Rashi and others explain that one way of interpreting the Gemara is that it refers to the day after the festival. According to this interpretation, the subject of the Gemara is someone who 'binds'/adds to the festival by celebrating the following day with food and drink.
Pardes Yechezkel - page 25 says that the day after Yom Tov is called Isru Chag, as we are supposed to tie up all the Mitzvos that we did over Yom Tov and remember them.