Where does “sunnundag” come from?

sunnundag (Old Saxon) comes from Proto-Germanic *Sunnōn dag, from Latin dies Solis, from Latin sōl, from Proto-Indo-European sóh₂wl̥ — sun.

Ancestry of “sunnundag”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	*Sunnōn dag	—
2	Latin	dies Solis	sun's day
3	Latin	sōl	—
4	Proto-Indo-European	sóh₂wl̥	sun

sunnundag — Etymology, Origin & Word History | Etymology Explorer