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.
I couldn't sleep well the night before last.
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).