The days of the week were created on different basis, as in french english even german and in japanese monday is the day of the moon get it? Moonday... 月、 げつ. In french we use the planets for the rest of the days of the week, and in japanese they use the chinese elements fire, water, wood, metal, and earth! It helped me a lot to remember it!
It's also the planets.
First there's the other two objects moving in the sky:
Sunday 日曜日 nichiyoubi - the Sun 日
Monday 月曜日 getsuyoubi - the Moon 月
Then it's 5 planets or the 5 chinese elements:
Tuesday 火曜日 kayoubi - Mars 火星 kasei (Fire-star)
Wednesday 水曜日 suiyoubi - Mercury 水星 suisei (Water-star)
Thursday 木曜日 mokuyoubi - Jupiter 木星 mokusei (Wood-star)
Friday 金曜日 kinyoubi - Venus 金星 kinsei (Metal-star)
Saturday 土曜日 doyoubi - Saturn 土星 dosei (Earth-star)