septembro

This is the tenth month to get a page, which means the set is nearly closed. Februaro, marto, aprilo, majo, junio, julio, aŭgusto and oktobro are already here, and januaro arrives in this same chapter. Only novembro and decembro are still outside, at ranks 2287 and 2019, which puts both of them past the core-2000 line - so if you want the last two months before the course reaches them, they are novembro and decembro, spelled exactly as you would guess.

Month names are ordinary nouns in Esperanto. They take -n and -j like anything else, they are written lower case in the middle of a sentence, and they need no preposition tricks: en septembro for the month, la kvinan de septembro for the date.

The counting mismatch in the etymology is the useful hook. Septem is seven, octo is eight, novem is nine and decem is ten, and the four months carrying those numbers sit at nine, ten, eleven and twelve. Learn the Latin numerals once and four Esperanto month names come free, provided you remember to add two.