Question: Matthew 12:40 says 3 nights, but there were only 2 – Friday and Saturday. Which is right?
Answer:

There are instances when Scripture records “sayings” or “concepts” that were common and well understood then, but usually are not well understood now.
Identifying and understanding their way of saying things is essential if we are to accurately understand and explain the Scripture. In the Lord’s day, it was customary that any part of the day is counted as the whole. One helpful resource for understanding these customs is the Jewish Talmud. The Talmud is a collection of laws, decisions, oral traditions and principles laid down by Jewish rabbis (beginning in the 8 century BC). It is a rich source of information regarding the customs and practices of the Jewish religious leaders and people during the 1st century AD. The Talmud informs us of the prevailing custom regarding what constituted a day and a night in Jesus day:
” …any part of the day is as the whole”
So, what did Jesus mean when He said he would be 3 days and 3 nights in the grave? Obviously He did not mean a literal 72 hours, as we understand it in the Western sense. Understanding the customary way of speaking, we can be assured that Jesus was speaking in a way that would be well-understood in His day, and frequently used! His hearers would have understood that any part of a day counted as the whole day. This sheds light on what Jesus meant!
All of us use the same kinds of “generalizations” in the course of our normal conversations today. For example, how many times have you said or heard something like this: “Boy, we worked day and night for a week to get the proposal done!” What you meant was that you spent a significant portion of your time on those things each day, but NOT that you “literally” worked for 168 hours on it!