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.
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
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.