John Gill Commentary Mark 14:2

"for they said, Not during the feast, lest haply there shall be a tumult of the people." — Mark 14:2 (ASV)

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).