two nights before last
Refers to the night two days before last night. A somewhat rare temporal noun; in everyday speech, people more often say 一昨日の前の夜 or use a phrase like 三日前の夜.
Two nights before last, it got very cold.
Standard kanji spelling for 'two nights before last'.
Variant spelling using the repetition mark 々; less common but still encountered.
Compound of 一昨 (いっさく, 'one before last') reduplicated to 一昨昨 (いっさくさく, 'two before last') + 夜 (や, 'night'). The repetition indicates an additional step back in time.