weekday
曜 centers on the concept of a weekday, specifically the seven-day cycle named after celestial bodies. It is used almost exclusively in the names of the days of the week.
曜 combines 日 (sun, day) with 翟, which likely contributes the sound and originally referred to a type of pheasant with bright plumage, suggesting radiance. The character was adopted to represent the shining celestial bodies used to name the days of the week.
The left side is 日 (sun, day), and the right side 翟 looks like a bird with feathers spread. Imagine a radiant bird marking each day of the week with its bright feathers, so 曜 means weekday.
For ヨウ, think of a yo-yo marking each day of the week: every weekday you play with a yo-yo. Yo-yo -> ヨウ.
—
Sunday
Sunday
Saturday
Friday
Saturday
Monday
Friday
Monday
Wednesday
Tuesday
Thursday
day of the week
Thursday
Wednesday
Tuesday
obsidian
Navagraha (divine personifications of the nine celestial bodies in Hindu mythology); nine luminaries
the seven luminaries (sun, moon, Mercury, Venus, Mars, Jupiter and Saturn)
the Big Dipper (asterism); the Plough; the Plow
calendar