Imagine spending two nights and three partial days in a cave by yourself and then returning to tell a friend, "I was in a cave for three days and three nights." How do you think your friend would react if he found out that you actually had only spent Friday afternoon through Sunday morning in the cave, and not a literal three full days and nights? He likely would think that you were lying.
Many people have wondered why Jesus once told a group of scribes and Pharisees that He would be in His tomb for "three days and three nights" (Matthew 12:40), if He was actually buried on Friday afternoon and rose from the grave early Sunday morning. The answer to this question lies in the fact that the expression "three days and three nights" that Jesus used does not mean that He literally was buried for 72 hours (24 hours x 3 days). If we think about Jesus' crucifixion, burial, and resurrection the way the first-century Jews did, expressions like "three days and three nights" (and similar phrases) are easily understood .
In Bible times, a part of a day frequently was spoken of as being equal to a whole day. During the United Kingdom period, when Israel went to King Rehoboam and asked him to lighten their burdens, he wanted time to think about their request, so he told the people to return to him "after three days" (2 Chronicles 10:5). Verse twelve of that chapter then says that the Israelites came to Rehoboam "on the third day, as the king had directed, saying, 'Come back to me the third day."' Even though Rehoboam instructed his people to return "after three days," they understood this to mean "on the third day."
Years later, when Queen Esther was about to risk her life by going before King Ahasuerus uninvited, she instructed her fellow Jews to follow her example by not eating "for three days, night or day" (Esther 4:16). But, the text goes on to tell us that Esther went in unto the king "on the third day" (5:1).
The New Testament writers also used various phrases (like "the third day," "after three days," and "three days and three nights") to describe how long Jesus' body was in the grave and on what day He rose. These phrases are different, but in that culture they all meant the same thing: Jesus was buried on Friday afternoon and rose on Sunday morning.