the day before yesterdaynnoun: Refers to person, place, thing, quality, etc.
(two days ago)
前天
The day before yesterday was my birthday.
the day before yesterdayadvadverb: Describes a verb, adjective, adverb, or clause--for example, "come quickly," "very rare," "happening now," "fall down."
(two days ago)
前天
I haven't seen him since the day before yesterday.