During Old Testament times, the Jews divided the night into three “watchers” or divisions of time (read Exodus 14:24). But after the Romans occupied their territory, they adopted the Roman pattern of the four-watch night.
Jesus referred to the four-watchers in His instructions to His disciples that they should always be ready for His return: the first (“even”) at three hours after sunset, the second three hours later at midnight, the third (“cockcrowing”) at three hours before sunset, and the fourth (“morning”) at dawn with the rising of the sun.
These were the same times when the Roman soldiers guarding Peter in prison rotated their shifts (read Acts 12:4).
MARK 13:35 – Watch ye therefore: for ye know not when the master of the house cometh, at even [in the evening. NIV], or at midnight, or at the cockcrowing [when the rooster crows, NIV], or in the morning [at dawn, NIV].