the day three days ago (three days before today); colloquially read as 'sakiototoi'.
一昨昨日（さきおととい）は久しぶりに友達と会った。
Three days ago I met a friend for the first time in a while.
事件は一昨昨日の夜に起きたと報告された。
The incident was reported to have occurred three nights ago.
さきおとといは雪が降って道路が渋滞していた。
Three days ago it snowed and the roads were congested.