Mark 14:17

And when it was evening he cometh with the twelve. And as they sat and were eating, Jesus said, Verily I say unto you, One of you shall betray me, even he that eateth with me.

And as they sat ... The Greek word here is "reclined."

And were eating ... Mark did not say, "eating the passover," but eating, that is, having a meal together the night before the paschal supper, in a room where preparations were only partially complete for the solemn beginning of Passover festival the next night.

One of you shall betray me ... John has a full account of the conversations and events leading up to this, but Mark abbreviated it. Judas, of course, was the one indicated. Regarding the prophetic identification of the traitor, see parallels in John and Matthew in this series.