Several parts combine into one character.

The traditional 晝 is a compound ideograph: 聿 (a brush held in hand) + 旦 (sunrise / morning, depicting the sun above the horizon). The original etymology suggests "the time when the brush divides morning from evening" — the day-bright period. Japan (shinjitai) and Mainland China (simplified) both adopted the same simplified form 昼; Korea retains the traditional 晝. Three regions, two scripts.

Korean reading "ju." 晝間 (jugan, daytime / day shift), 晝夜 (juya, "day and night" — used for round-the-clock operations), 白晝 (baekju, "broad daylight" — used in idioms like 백주에 in broad daylight). Korean preserves the traditional 晝 in formal writing, though everyday Korean uses the native (nat) for "daytime."

Mandarin zhòu, 4th tone. 白昼 (báizhòu, daytime — formal), 昼夜 (zhòuyè, day and night — formal), 昼伏夜出 (zhòufú yèchū, "hide by day and emerge by night" — describes nocturnal animals or stealth operations). However, modern colloquial Chinese strongly prefers 白天 (báitiān, "white sky" = daytime) for everyday speech; 昼 is reserved for literary and idiomatic register.

Japanese keeps 昼 actively in everyday vocabulary. On-reading チュウ (chū) — 昼食 (chūshoku, lunch — formal), 昼夜 (chūya, day and night). Kun-reading ひる (hiru) is what Japanese speakers use constantly: 昼 (hiru, noon / midday / lunch — meaning all three), 昼ご飯 (hirugohan, "noon rice" = lunch — extremely common), 昼休み (hiruyasumi, "noon break" = lunch break / midday rest), 昼間 (hiruma, daytime). When Japanese say "let's eat 昼 (hiru)," everyone understands lunch. The character anchors all of Japan's midday vocabulary.

Memory aid: brush dividing the day from night — 晝 traditional, 昼 simplified. Korea uses 晝, Japan and Mainland China use 昼.