Where does “Σεπτέμβριος” come from?

Σεπτέμβριος (Koine Greek) comes from Latin september, from Latin Septem, from Proto-Italic septem, from Proto-Indo-European septḿ̥ — seven.

Ancestry of “Σεπτέμβριος”, 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

Σεπτέμβριος — Etymology, Origin & Word History | Etymology Explorer