1The day before yesterday — two days prior to today. Read as 'おととい' or 'おとつい' in everyday speech, and sometimes as the more formal 'いっさくじつ' in written Japanese.
一昨日、駅前で高校の同級生に偶然会った。
The day before yesterday, I happened to run into a high school classmate in front of the station.
一昨日から頭が痛くて、今日は病院に行くつもりだ。
I've had a headache since the day before yesterday, so I plan to go to the hospital today.
一昨日は休みだったので、一日中家でゆっくりしていた。
The day before yesterday was a holiday, so I relaxed at home all day.
一昨日提出された報告書には誤りが見つかった。
An error was found in the report submitted the day before yesterday.