Several parts combine into one character.

曜 is a phono-semantic character: 日 (sun) for meaning and 翟 (a pheasant) for sound. Its original sense was "to shine, the light of sun and stars" — from which it came to denote the sun, moon, and five planets (the Seven Luminaries), and then the names of the days of the week.

The East Asian week, the 七曜 (Seven Luminaries), is the seven lights of sun, moon, Mars, Mercury, Jupiter, Venus, Saturn — 日曜 (Sun), 月曜 (Moon), 火曜, 水曜, 木曜, 金曜, 土曜. This seven-day cycle began in ancient Babylon, passed through India and China, and settled across East Asia — a globally shared week.

Korean hanja-sound "yo": 曜日 (yoil, day of the week), 七曜 (chiryo, the seven luminaries), 日曜日 (iryoil, Sunday). In Korean the days are usually written ~曜日, as in woryoil (Monday), hwayoil (Tuesday).

Mandarin: yào, 4th tone. In modern Chinese the days of the week are counted with 星期 (xīngqī, from the 星 entry seen earlier), so 曜 is rare, met mostly in Japanese loanwords or classical and literary use — 日曜 (rìyào).

Japanese: on-reading ヨウ (yō) — no kun-reading (on-only). 曜日 (yōbi, day of the week), 月曜日 (getsuyōbi, Monday), 日曜日 (nichiyōbi, Sunday), 土曜日 (doyōbi, Saturday). Japanese days of the week all end in ~曜日 (yōbi).

Memory aid: the sun (日) shining (翟) — from the seven lights (七曜) to the days of the week.