Where does “土曜日” come from?

土曜日 (Japanese) comes from Japanese 日, from Old Japanese 日, from Proto-Japonic *pi.

土曜日 (Japanese): Saturday

Definitions

Step	Language	Word	Meaning
1	Japanese	日	Short for 日曜日 : Sunday; the sun; sunlight,...
2	Old Japanese	日	the sun; sunlight, sunshine; daytime, daylight
3	Proto-Japonic	*pi	—

土曜日 — Etymology, Origin & Word History | Etymology Explorer