Midnight, literally "the middle of the night", was a time arbitrarily designated to determine the end of a day and the beginning of the next in some, mainly Western, cultures. Originally midnight depended on the time of the sunset and dawn, varying according to the seasons. With the advent of clocks and the division of the day into twenty-four hours, midnight was established at 12 AM (0:00 hrs).
When relevant conventions were made, it was officially established that midnight was going to be the moment of the end of one day and the beginning of the next. This convention has been generally accepted for international purposes (although some peoples continue their tradition of ending and beginning a day at another time for religious purposes, i.e. in Judaism).
In Western tradition midnight is the time at which spirits, ghosts, demons and other supernatural entities prefer to appear, and also the time to begin the Sabbath. This is mainly due to the fact that many Heathen festivals, which Christianity turned into demonic, began at midnight.
However, all reputable systems consider midnight to be the next day, and NOT the day prior.