They're preparing to share the Passover meal.
The passage from Exodus reminds of the directions for the first Passover. The Lord had told them that on the night before they were to escape slavery in Egypt, the entire congregation of Israel was to assemble in order to slaughter lambs for a meal. Each household would have a lamb and include neighbors whose household was too small for its own lamb.
And the passage reminds of what is remembered as they continue to gather--their rescue. After the supper, a plague had come through Egypt striking down the first-borns. Only Israel had been saved.
Jesus knows that on this Passover, his hour has come. He knows of his upcoming death and of the betrayal by one close to him.
On this last Passover, with the memory of what happened on the first one and what has happened to his people since, Jesus chooses to wash the feet of his disciples.
Peter considers this unseemly, but Jesus insists, "You'll understand later."
We, the church, are living in the later. I'm wondering which is harder to understand--that I am to allow Jesus to stoop down and perform the work of a lowly servant, or, that Jesus is asking me to emulate him, that I'm expected to stoop down and perform servant work.
Jesus tells them how his disciples will be recognized. They will be the ones who have love for one another.