Why is the ninth month called September?
word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha-, from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins] (cf. Sanskrit thah, Greek -tos, Latin -tus).
suffix forming nouns (e.g. depth, strength, truth, etc.), from Old English -ðu, -ð, from Proto-Germanic *-itho, abstract noun suffix, from PIE *-ita (cf. Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free").