Matthew 16
A demand for a sign
The Pharisees also with the Sadducees came, and tempting desired him that he would shew them a sign from heaven. He answered and said unto them, When it is evening, ye say, It will be fair weather: for the sky is red. And in the morning, It will be foul weather to day: for the sky is red and lowring. O ye hypocrites, ye can discern the face of the sky; but can ye not discern the signs of the times? 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.
Pharisees and Sadducees test Jesus by asking for a sign from heaven. He replies that they can read the weather from the colour of the sky but not the signs of the times, and repeats that this generation will get no sign except the sign of Jonah. Then he leaves them.
What does this mean?
A 'sign from heaven' meant an unmistakable proof from God.
Historical context
Pharisees and Sadducees disagreed on much, including resurrection and the authority of tradition; Matthew shows them joining forces against Jesus. A red evening sky as a sign of fair weather is ancient weather lore found in many cultures.
Why does it matter?
The words about reading the sky are missing from some early manuscripts, though both the KJV and WEB include them. The repeated 'sign of Jonah' (12:39) points ahead to Jesus' death and resurrection.
Editorial notes, not scripture.