昨
サク

yesterday, previous

JLPT3 G4 S9 F226

As per 乍# (create a pile) + 日 sun/day → figurative pile of days (compare 昔 and 時) → *past*; *day/year before*. 昔 and 昨 likely were variants of each other that eventually diverged in usage.

[Full Kanji Etymology List]