But they said not on the feast day,.... The feast of the passover, and of unleavened bread, which was nigh at hand, and would be two days hence, when there would be a great concourse of people from all parts to keep it: and therefore they did not choose to seize him, and put him to death at that time,
lest there should be an uproar of the people; or among them, lest they should rise in his favour, and rescue him out of their hands; See Gill on Matthew 26:5.

But they said, Not on the feast day--rather, not during the feast; not until the seven days of unleavened bread should be over.
lest there be an uproar of the people--In consequence of the vast influx of strangers, embracing all the male population of the land who had reached a certain age, there were within the walls of Jerusalem at this festival some two million people; and in their excited state, the danger of tumult and bloodshed among "the people," who for the most part took Jesus for a prophet, was extreme. See JOSEPHUS [Antiquities, 20.5.3]. What plan, if any, these ecclesiastics fixed upon for seizing our Lord, does not appear. But the proposal of Judas being at once and eagerly gone into, it is probable they were till then at some loss for a plan sufficiently quiet and yet effectual. So, just at the feast time shall it be done; the unexpected offer of Judas relieving them of their fears. Thus, as BENGEL remarks, did the divine counsel take effect.
The Supper and the Anointing at Bethany Six Days before the Passover (Mark 14:3-9).
The time of this part of the narrative is four days before what has just been related. Had it been part of the regular train of events which our Evangelist designed to record, he would probably have inserted it in its proper place, before the conspiracy of the Jewish authorities. But having come to the treason of Judas, he seems to have gone back upon this scene as what probably gave immediate occasion to the awful deed.