After Jesus rebuked the scribes and Pharisees for asserting that He worked His miracles via the power of the devil, they asked to see a sign from Him. There are perhaps two primary reasons for this request. First, they wanted to appear to the multitude to be open-minded, though in actuality they wanted to test Jesus. Second, they were, of course, displeased by His harsh words against them and wanted a miracle or some manifestation of divine authority to back up His words.
Jesus responded with another rebuke, as He labeled them "an evil and adulterous generation". The nation of Israel was sometimes represented as being married to God (e.g., Jer. 3:14,20). Every time they forsook His covenant, they committed adultery in a spiritual sense. It is likely that Jesus labeled them as "evil" for two reasons: (1) their motive in asking for a sign was not pure (they were not seeking evidence in order to believe but looking for things to criticize), and (2) their eyes were not set on seeing the truth but on seeing their prestige and influence reestablished with the people.
Our Lord then declared - "No sign will be given...except the sign of the prophet Jonah." Jesus only provided His listeners with a veiled explanation of this phrase, although the meaning is obvious today. Jonah was shown to be a true prophet of God, and Ninevah received him as such because he was rescued from the fish's belly (cf. Luke 11:30). In a similar manner, Jesus would be declared to be the Son of God via His resurrection from the dead (Rom. 1:4). As Jonah came forth alive after three days in a fish, so also would Jesus come forth alive after three days in a tomb. This would be the great sign Jesus would provide for His skeptics--and He did just that!
Interestingly, Jesus here affirmed that He would be buried for "three days and three nights". This detail has raised some serious chronological questions for careful students of the Bible. It is generally understood that Jesus died on a Friday afternoon and was risen from the dead early Sunday morning. However, that time frame certainly doesn't fulfill the notion of "three nights" as humans currently speak regarding time. We will share the solution to this alleged contradiction tomorrow as we continue studying this context.