Several parts combine into one character.

Phonetic-semantic compound: 日 (sun / day) + 乍 (suddenly / momentarily). The encoded meaning: "the day that just barely passed" = yesterday. Identical across 繁體 / 新字体 / 简体.

Mandarin: zuó, rising 2nd tone. 昨天 (zuótiān, yesterday — the everyday word), 昨晚 (zuówǎn, last night), 昨日 (zuórì, yesterday — formal/written), 昨夜 (zuóyè, last night — formal/poetic). The pair 昨天/明天 (yesterday/tomorrow) is foundational time vocabulary, taught alongside 今天 (today).

Japanese: on-reading サク (saku) for compounds — 昨日 (sakujitsu, yesterday — formal), 昨年 (sakunen, last year — formal), 昨夜 (sakuya, last night — formal). However — and this is critical — the everyday Japanese word for "yesterday" uses an irregular kun-reading hybrid: — 昨日 can be read さくじつ (sakujitsu, formal) OR きのう (kinō, casual / everyday). — The reading きのう is one of the most common irregular kanji compounds in Japanese, reflecting an older Japanese word that got assigned to 昨日 in writing.

This split — formal sakujitsu vs. casual kinō — is among the foundational duplicate-reading patterns Japanese learners must memorize. In speech, きのう dominates; in news and formal writing, さくじつ appears.

The related word for "tomorrow" — 明日 — has the same dual structure: あした (ashita, casual) or あす (asu, slightly formal) or みょうにち (myōnichi, very formal). Three different readings for the same kanji compound, reflecting register.

Memory aid: sun + a moment ago = yesterday.