אבן עזרא — ביום השמיני... עצרת הִוא כל מלאכת עבדה לא תעשו

Q — What does "עצרת" mean? Onkelos translates "כנישין תהון" — an assembly. Yonatan ben Uziel on "עצרת בוגדים" (Yirmiyahu) translates "סיעת שקרין" — a band, gathering. So "עצרת" should mean "assembly," and the eighth day of Sukkot would be the assembly day of all Israel at the regalim.

A — Ibn Ezra rejects the "assembly" reading on textual grounds: Pesach also has "וביום השביעי עצרת" (Devarim 16:8), but the very preceding pasuk says "ופנית בבקר והלכת לאהליך" — they've already dispersed! The seventh day of Pesach can't be an "assembly day" if everyone has gone home. Ibn Ezra's reading: "עצרת" derives from "נעצר לפני ה'" (Shmuel I 21:8 about Doeg), meaning "halted from all worldly business" — devoted exclusively to avodat Hashem. The pasuk then self-explains: "עצרת הִוא — כל מלאכת עבודה לא תעשו." The same pattern holds in all three Torah occurrences of עצרת (Pesach, Shavuot/Atzeret, Shmini Atzeret) — the term is consistently followed by the prohibition on work, the Torah glossing its own term. The methodological move parallels Ibn Ezra's "בית הסהר" reading in Vayeshev: when the Torah explains a term, the explanation IS the meaning.