The NAB's note on Mark 14:26 -- "Then, after singing a hymn, they went out to the Mount of Olives." -- reads, "Psalm 114-118, thanksgiving songs concluding the Passover meal."
Christians have always read the Psalms as prefiguring Jesus, but what must it have been like for Jesus Himself to sing these psalms of deliverance with His disciples mere hours before they were to be fulfilled? Did the disciples have any idea that the words they sang were not mere recollections of the past events but prophesy of the present?
How did the words of Psalm 116 sound in that upper room?
I was caught by the cords of death; the snares of Sheol had seized me; I felt agony and dread. Then I called on the name of the LORD, "O LORD, save my life!"
And what about Psalm 115's taunt of Israel's enemies, that describes their idols this way:
They have mouths but do not speak, eyes but do not see. They have ears but do not hear, noses but do not smell. They have hands but do not feel, feet but do not walk, and no sound rises from their throats.
Here singing these words is Jesus, Israel's God, Who has a mouth that speaks, eyes that see, ears that hear, a nose that smells, hands that feel, feet that walk, a throat from which sound comes. Before the sun sets again, His mouth will speak of betrayal, His eyes will see abandonment, His ears hear blasphemy, His nose smell His own blood, His hands feel the nails, His feet walk under the cross, and from His throat will come a loud cry and His final breath.
Afterwards, His body will be as the idols of the nations.
And, for all that, it will be the source of life for us all.