It depends on your point of reference. If you are talking about today, the previous day would be yesterday, and the next day would be tomorrow.
Now imagine you're telling a story about a specific day in the past, your birthday last year, for example. You might want to talk about what happened the day before your birthday, the previous day, or what happened the day after your birthday, the following day. In this case you could not use the words "yesterday" or "tomorrow".