Etymologically, it's the day of wood, one of the five elements in Chinese/Eastern philosopy (fire, water, wood, metal and earth). But as KeithWong9 points out, the planets were also named after the elements and it is more likely that the weekdays were named after them than directly after the elements (since there are also 日曜日 and 月曜日).
This looks hard, but don't be scared... the Japanese week is just like our English week! I'll show you.
▪日曜日 / にちようび / "Sun Day" / Sunday, of course! ▪月曜日 / げつようび / "Moon Day" / Monday
(Those two especially sound familiar, right?)
The days starting with Tuesday use the kanji for the 5 elements and correspond to our calendar.
▪火曜日 / かようび / "Fire Day" / Tuesday ▪水曜日 / すいようび / "Water Day" / Wednesday ▪木曜日 / もくようび / "Wood Day" / Thursday ▪金曜日 / きんようび / "Metal Day" / Friday ▪土曜日 / どようび / "Earth Day" / Saturday
Once I learned this, I never forgot the days of the week again! Cool, right?
I think one way (at least for the Kanji) is to think of them as increasing in weight: 火: Tuesday (fire) 水: Wednesday (water) 木: Thursday (wood) 金: Friday (gold/metal) 土: Saturday (soil/earth)
Though metal is typically heavier/denser than soil, the connection of soil to the Earth makes it weightier.
At least this way of thinking about it works for me.