noun, adverb
the night before last
Refers to the night two nights ago. Used as a noun or adverb. More formal than おとといの夜.
一昨夜はよく眠れなかった。
I couldn't sleep well the night before last.
一昨夜の出来事を覚えていますか。
Do you remember what happened the night before last?
More common and casual way to say 'the night before last'.
Compound of 一昨 (いっさく, 'one before last') and 夜 (や, 'night'). The reading いっさく is a phonetic shift from 一昨 (いっさく).