It's kind of interesting, or familiar, that these fleet-footed, overreaching Religionists would come (spurred on by Satan, most likely: the ultimate tempter!) and "tempt" Him, after He had performed all of these miracles, and it's also somewhat strange (but interesting) that the Pharisees "with" the Sadducees had come together. The Pharisees were of the 'extreme' sort, of course, but the Sadducees were unitarians; they didn't believe in the 'supernatural' or the resurrection, which is kind of ironic, considering all of the supernaturalism that had been advancing and overlapping around them at that time!
They came asking for a sign from heaven, in the hopes that Christ would truly show them something, apparently, because they "needed a sign" (the Jew always require a sign, right? Ha!), but Christ goes on to respond to their desires (I would imagine that they may have asked Him to do these things, almost with a sense of pomp in their voices; or, at least, a kind of audible annoyance? or sarcasm? Granted, they themselves wanted Him to do something for them, specifically ['show us a sign from heaven!']).
In verse 3, it says:
And in the morning, It will be foul weather to day: for the sky is red
and lowering. O ye hypocrites, ye can discern the face of the sky; but
can ye not discern the signs of the times?
Obviously Christ was holding them accountable for not recognizing Who they were even talking to (hadn't they read their Scriptures?!), and by "signs of the times," he's referring to the signs of the 'end', to realize that The Messiah had come, and the fact that He had come at that time should have been something that they recognized was in God's perfect timing!
It goes on in verse 4, which is interesting, too:
A wicked and adulterous generation seeketh after a sign; and there
shall no sign be given unto it, but the sign of the prophet Jonas. And
he left them, and departed.
I bet after he 'departed', they were probably scratching their heads, thinking, "What in the world is He talking about?" Or, could it be that Christ references the prophet Jonah to get them to think, to remember the passages, to consider what it means? In previous chapters, Jesus mentions Jonah again (chapter 12). "Think on this, guys!"
Jesus Christ is obviously referring to Jonah's spending the three days and three nights in the belly of the whale, and using that as an example to say, "So will I, but it will be three days and three nights in the belly of the earth!" (referencing His Resurrection, of course).
There are certain commentators that make the 'second fulfillment' of what Jesus is referring to in light of Israel as a Nation: how it will be a sign to them when, idiomatically in the future when the remnant pleas for the Messiah to return and save them, which of course Christ does at the end of the Time of Jacob's (Israel) Trouble, when he returns "after the third day" (hence the parallels to this in Matthew 16:4) and steps in to end it, and then set up his Kingdom here the "new" replenished Earth and henceforth reign for 1,000 years.