As a Noun: 1. The day before today. English = yesterday. 2. The day before a particular day in the past. English = the day before, the previous day. As Modifier: 1. Used before or after a sign sentence to say the action in the sentence happened during the day before today. If you are already talking about something that happened on a day before today, then you mean it happened during the day before that. 2. English = yesterday; the day before, the previous day.