Kluger:
Shmini Atzeres is not acharon shel sukkos. It’s its entirely own holiday. However, I will admit that I’ve erroneously explained it as the last days of sukkos to my goyish employers in the past because it’s just easier to explain that way. Hosanah Rabbah is not a chag. Shushan Purim is not, by any stretch of the imagination, “the last day of Purim.”
Pesach is the only holiday with an acharon. How is this thread still going? Why complicate something so extremely pashut?