曜日
ようび · youbi · JLPT N3 · noun
day of the week
📅 曜日 = luminary + day: each weekday is named after a planet
曜 means a shining heavenly body. The seven days follow the sun, moon and five planets: 日曜日 is Sunday (sun), 月曜日 Monday (moon), 火曜日 Tuesday (Mars, the fire star). This is the same pattern English uses for Sun-day and Mon-day.
日 — pictograph of the sun
日 is a pictograph of the sun, originally drawn as a circle with a dot or stroke inside representing the sun's substance. Over time the round shape was squared off into the modern rectangular form. It means 'sun' and by extension 'day'.