Simple: Monday = Moon = 月 Tuesday = Mars = 火星 Wednesday = Mercury = 水星 Thursday = Jupiter = 木星 Friday = Venus = 金星 Saturday = Saturn　土星 Sunday = Sun = 日 That should clear that up. BTW, the five elements （五行、火水木金土）of TCM are somehow related to the planets, too.

Monday............Getsuyoubi........ 月曜日 moon The moon, Tsuki (月) Tuesday...........Kayoubi.......... 火曜日 fire Mars, Kasei (火星) Wednesday......Suiyoubi.........水曜日 water Mercury, Suisei (水星) Thursday..........Mokuyoubi....... 木曜日 wood Jupiter, Mokusei (木星) Friday..............Kin'youbi.........金曜日 metal Venus, Kinsei (金星) Saturday..........Doyoubi.......... 土曜日 earth Saturn, Dosei (土星) Sunday............Nichiyoubi......日曜日 Sun The sun, hi (日); taiyou (太陽)

Dates were related to oriental ( India, now of China, Tibet,.) philosophies. Our dates weren't counted as it is of now. Each days were supposedly related by the philosophies and religions in ancient times. So does the Kanji which was brought from China (practically from Korea to Japan because our Kanji is the one brought from there). But there too many things are mixed up by our Japanese culture now and then. Altho, I think why do we use these Kanji is something related about the philosophies should explain your question: "could you tell why do week days mean day of: fire, water, tree, gold and earth." If you need to know the rearsons why, you may want to ready to be a monk ^^