What the text says. The command is in Leviticus: "On the fifteenth day of this seventh month is the feast of tabernacles for seven days unto Jehovah" (Leviticus 23:34, ASV), and "Ye shall dwell in booths seven days" (Leviticus 23:42, ASV). John dates a whole chapter by it: "Now the feast of the Jews, the feast of tabernacles, was at hand" (John 7:2, ASV).
What the week looked like, from the Mishnah. The Mishnah, the rabbinic law code compiled about AD 200, describing Temple practice from the memory of the generations after it fell, devotes a tractate, Sukkah, to this feast, and its detail is domestic and specific. A water libation was performed each day of the seven: "A golden flask holding three logs was filled from the Shiloah. When they arrived at the water gate, they sounded a teki'ah [long blast], a teru'ah [a staccato note] and again a teki'ah" (Sukkah 4:9). At night the Temple was lit: at the close of the first festival day they set up "golden candlesticks … and four youths drawn from the young priests, and in their hands there were jars of oil containing one hundred and twenty logs which they poured into the bowls", and, the line that describes the whole city, "there was not a courtyard in Jerusalem that was not illuminated by the light of the Bet Hashoevah" (Sukkah 5:2-3). Of the celebration the tractate says: "he who has not seen the Simchat Bet Hashoevah has never seen rejoicing in his life" (Sukkah 5:1).
The connection to two sayings, an argument, labelled as one. John places two sayings in this week. On "the last day, the great [day] of the feast", Jesus cries, "If any man thirst, let him come unto me and drink" (John 7:37, ASV); shortly after, in the same Temple setting, "I am the light of the world" (John 8:12, ASV). Many commentators read both against the water ceremony and the lamps, and the fit is close. ⚠️
But two cautions are owed. First, John does not make the connection himself, the Gospel supplies the date, and the correspondence is the reader's inference; it is an argument about setting, not a statement of the text. Second, the Mishnah is later evidence: compiled around AD 200, it describes a Temple destroyed in AD 70, so its detail is recollection, and how exactly first-century practice matched it cannot be settled from it alone. The dating in John and the ceremonies in Sukkah are each solid; the bridge between them is an inference, and worth having as one.
Sources: Leviticus 23:34, 23:42-43; Deuteronomy 16:13; John 7:2, 7:37; John 8:12; Nehemiah 8:14-17; Zechariah 14:16; Mishnah Sukkah 4:9 and 5:1-4; ISBE 1915, "Feasts and Fasts".
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