"Now it came to pass, when Jesus had finished all these sayings, that He said to His disciples, 'You know that after two days is the Passover, and the Son of Man will be delivered up to be crucified.' Then the chief priests, the scribes, and the elders of the people assembled at the palace of the high priest, who was called Caiaphas, and plotted to take Jesus by trickery and kill Him. But they said, 'Not during the feast, lest there be an uproar among the people'" (Matt. 26:1-5).
Jesus had spoken a great deal regarding the destruction of Jerusalem and the end of the world in Matthew 24 & 25, but now those sayings were "finished." Some have speculated that Jesus now proceeded to make His way back to Bethany, where He would stay that night and Wednesday.
"You know that after two days is the Passover, and the Son of Man will be delivered up to be crucified" - The disciples were certainly familiar with the fact that the Passover was quickly approaching (as He spoke these words, it was probably after sunset; thus, the Jews reckoned it as Wednesday which was two days away from Friday). However, the disciples certainly didn't understand the fact that Jesus would soon be crucified. Although He had prophesied such on several occasions, they were slow to understand this matter.
The description regarding the rulers gathering to plot Jesus' death seems to be that of a formal meeting, though perhaps private. The court where they met on that occasion was an area enclosed by the palace of the high priest. Their purpose in assembling was simple. They desired to find a way "to take Jesus by trickery and kill Him." Evidently it was hoped that their gathering together would help them formulate a plan of attack.
"But they said, 'Not during the feast, lest there be an uproar among the people'" - These men agree that Jesus must be arrested and killed, but they decide that such would not be a wise thing for them to do during the feast. The reason why is because the only way they know how to arrest Jesus is publicly, since His whereabouts in the evenings had been kept secret. They know that publicly taking Him by force could lead to disaster. Based on the public display of support during Jesus' triumphant entry (two days earlier), they cannot risk a revolt that could be stimulated by a public arrest. So, they decide to wait until after the feast, when it would be safer. By that time much of the current excitement would have died down, and those from Galilee and other regions would have returned home, thereby making a rebellion less likely when they arrest Jesus.