1º_ Day immediately prior to today. See tomorrow ("day after today"), "before yesterday" ("day before yesterday"), "day after tomorrow", "day after tomorrow". 2º_ By the previous one, it is said of the past tense, and is usually used as the locution "yesterday".
Day before today, day that has passed. It means day that has passed, day that has passed, this previous one. Past, moments that happened. History, memories, once, before.