Where does “Martis dies” come from?

Martis dies (Latin) comes from Latin Mārs, from Proto-Indo-European -ḱos — -y, -ic.

Ancestry of “Martis dies”, step by step

Step	Language	Word	Meaning
1	Latin	Mārs	—
2	Proto-Indo-European	-ḱos	-y, -ic

Martis dies — Etymology, Origin & Word History | Etymology Explorer