昨 is almost always used in 昨天 (yesterday) and 昨晚 (last night). Contains 日 (sun/day) — relating to time. Compare the time words: 前天 (day before yesterday), 昨天, 今天, 明天, 后天.
Sun radical on the left, the indexing radical here. 昨 means yesterday, a unit of time measured in suns — placing it in the day family with 明 tomorrow, 早 morning, 晚 late, 时 hour. The semantic link is straightforward: a past day.
Right side 乍 supplies the sound — zhà drifting to zuó through a regular zh/z alternation. 乍 itself means suddenly or just now, and that whiff of recent-past time reinforces 昨 nicely. Same phonetic appears in 作 to do, 炸 explode, 蚱 grasshopper.
