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?
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).