"The day before yesterday" refers to the day just before yesterday, or two days prior to the present day. There are a variety of synonyms for this phrase that can be used interchangeably. Some of the commonly used alternatives include "two days ago," "the other day," "yesterday's yesterday," and "the second day before today." Additionally, depending on the context and level of formality, you could use other expressions like "the preceding day," "the day earlier," or simply "two days in the past." No matter which synonym you choose, using these phrases can help you be more descriptive and specific when talking about past events.