The day before today. It refers to the 24-hour period preceding the present day. Often used when recounting past events or experiences, marking what has already occurred.

今日(こんにち)の前(まえ)の日(ひ)、つまり今日(こんにち)から数(かぞ)えて24時間(じかん)前(まえ)の日(ひ)のことです。過去(かこ)の出来事(できごと)を話(はな)す時(とき)に使(つか)います。