From Longman Dictionary of Contemporary Englishthe day/week/year etc before lastthe day/week/year etc before lastBEFOREthe day, week etc before the one that has just finished I sent the letter off the week before last. → last
Examples from the Corpus
the day/week/year etc before last• In the week before last, claims rose by 22,000.• The Sunday newspaper articles had come out the week before last, and were still bringing in letters.• I didn't know myself where the house was until the week before last. 