On the eighth day you shall have an assembly; you shall do no laborious work.
Torah Temimah: Chagigah 9a
English: One who did not celebrate on the first Yom Tov of the festival may celebrate throughout the festival and on the last Yom Tov of the festival. From where? R. Yochanan said in the name of R. Shimon b. Yehotzadak: “atzeres” is said of the seventh of Pesach and of the eighth of the festival.
Makeup window for chagigah.
Shemini Atzeres closes the opportunity to bring missed festival offerings.