Where does “сентя́брь” come from?

сентя́брь (Russian) comes from Latin september, from Latin Septem, from Proto-Italic septem, from Proto-Indo-European septḿ̥ — seven.

Ancestry of “сентя́брь”, step by step

Step	Language	Word	Meaning
1	Latin	september	of September
2	Latin	Septem	—
3	Proto-Italic	septem	seven
4	Proto-Indo-European	septḿ̥	seven

сентя́брь — Etymology, Origin & Word History | Etymology Explorer