On the first day of unleavened bread, when they sacrificed the Passover, his disciples asked him, “Where do you want us to go and prepare that you may eat the Passover?”
He sent two of his disciples and said to them, “Go into the city, and there a man carrying a pitcher of water will meet you. Follow him, and wherever he enters in, tell the master of the house, ‘The Teacher says, “Where is the guest room, where I may eat the Passover with my disciples?”’ He will himself show you a large upper room furnished and ready. Get ready for us there.”
His disciples went out, and came into the city, and found things as he had said to them, and they prepared the Passover.
When it was evening he came with the twelve. As they sat and were eating, Jesus said, “Most certainly I tell you, one of you will betray me—he who eats with me.”
They began to be sorrowful, and to ask him one by one, “Surely not I?” And another said, “Surely not I?”
He answered them, “It is one of the twelve, he who dips with me in the dish. For the Son of Man goes as it is written about him, but woe to that man by whom the Son of Man is betrayed! It would be better for that man if he had not been born.”
On the day the Passover lambs are sacrificed, Jesus sends two disciples into the city to follow a man carrying a water jar to a house with a large upper room, where they prepare the meal. That evening, as they eat, Jesus says that one of the Twelve eating with him will betray him.
What does this mean?
The 'goodman of the house' (KJV) is the master of the house; a 'guestchamber' is a guest room. 'Dippeth with me in the dish' refers to sharing a common bowl, dipping bread into it.
Historical context
Passover lambs were slaughtered at the temple in the afternoon and eaten that evening with unleavened bread, bitter herbs and wine. It is often suggested that a man carrying water, usually women's work, would have been easy to spot. In Mark, as in Matthew and Luke, the Last Supper is a Passover meal; in John, Jesus dies as the lambs are being slaughtered. Scholars have proposed various explanations for the difference.
Why does it matter?
Betrayal by someone sharing one's table was an especially deep violation, echoing Psalm 41:9. 'As it is written' presents Jesus' death as part of God's purpose, while the 'woe' holds the betrayer responsible.
Editorial notes, not scripture.