"Back in the day" is a saying that simply refers to the past. It is often associated with fond memories and nostalgia of how things once were. Typically it refers to many years in the past and never to recent events. When used in speech, it's usually specific to the individual's personal history, or a group of friends who were closely associated to one another. Many people refer to their childhood or college years as "back in the day," however it's important to note that some day in the not too distant future, right now will be "back in the day."