日 is one of the oldest surviving pictographs in the CJK script family. Oracle bone inscriptions show a circle enclosing a single dot — early scribes may have observed sunspots without telescopes. Over the centuries the curve hardened into a rectangle and the dot survived as the inner stroke. The character is identical in 繁體 (Traditional), 新字体 (Japanese shinjitai), and 简体 (Simplified Chinese) — no reform ever touched it.

Japanese splits the on-reading by register: ニチ (nichi) for everyday compounds — 日記 (nikki, diary), 日曜日 (nichiyōbi, Sunday) — and ジツ (jitsu) for stiffer ones like 平日 (heijitsu, weekday) and 祭日 (saijitsu, holiday). The kun-reading ひ (hi) keeps the literal sun: 朝日 (asahi, morning sun). The country name 日本 reads にほん or にっぽん, literally "origin of the sun" — the land east of China where the sun rises.

Mandarin: rì, a clean falling 4th tone. Modern spoken Chinese rarely uses 日 alone for "day" — 天 (tiān) wins there — but 日 dominates compounds: 生日 (shēngrì, birthday), 日记 (rìjì, diary), 日本 (Rìběn, Japan), 节日 (jiérì, holiday).

Memory aid: a window-shaped box with one horizontal stroke inside. If you can draw a window, you can draw 日.