I'm not sure if this answer is correct but it seems to be true.

Sunday: sun（太陽「たいよう」＝日）→日曜日

Monday: moon（月「つき」）→月曜日

Tuesday: the day for "Tiu" (Mars) （火星「かせい」）→火曜日

Wednesday: the day for "Woden" (Mercury)（水星「すいせい」）→水曜日

Thursday: I have no idea

Friday: the day for "Frigg" (Venus)（金星「きんせい」）→金曜日

Saturday: the day for "Saturn"（土星「どせい」）→土曜日

I guess the name of the days in Japanese is basically based on the name of stars, which is same to English. I hope it helps you study Japanese.