“But about that day or the hour no one knows, neither the angels in heaven, nor the Son, but only the Father. Keep awake, watch; for you do not know when the time will come. It is like a man going on a journey, when he leaves home and puts his slaves in charge, each with his work, and commands the doorkeeper to be on the watch. Therefore, watch—for you do not know when the master of the house will come, in the evening, or at midnight, or at cockcrow, or at dawn, or else he may find you asleep when he comes suddenly. And what I say to you I say to all: Watch.” (Mark 13:32-37)
Observe: “in the evening, or at midnight, or at cockcrow, or at dawn…” — not 24 hours, but about 12, through the watches of the night. Jesus’ last night is about to run right through that sequence:
(1) Jesus betrayed at the Last Supper at evening: “When it was evening, he came with the twelve. And when they had taken their places and were eating, Jesus said, ‘Truly I tell you, one of you will betray me, one who is eating with me.’” (Mark 14:17-18)
(2) Jesus finds the disciples “sleeping” in Gethsemane, having failed to keep “watch” (cf. Mark 13:36-37, end of the above), presumably at midnight, when “the hour” (cf Mark 13:32!) comes:
“He came and found them sleeping; and he said to Peter, ‘Simon, are you asleep? Could you not watch one hour?'” (Mark 14:37)
“He came a third time and said to them, ‘Are you still sleeping and taking your rest? Enough! The hour has come; the Son of Man is betrayed into the hands of sinners. Get up, let us be going. See, my betrayer is at hand.’” (Mark 14:41-42)
(3) Peter denies Jesus at cockcrow: “And immediately the cock crowed a second time. And Peter remembered how Jesus had said to him, ‘Before the cock crows twice, you will deny me three times.’ And he broke down and wept.” (Mark 14:72)
(4) Jesus handed over to Pilate at dawn: “As soon as it was dawn, the chief priests held a consultation with the elders and scribes and the whole council. They bound Jesus, led him away, and handed him over to Pilate.” (Mark 15:1)
Mark’s literary-theological point: In the “Parable of the Doorkeeper” above (Mark 13:32-27), Jesus tells the disciples several times over to “watch” and not be found sleeping precisely because they do not know “the hour,” which could come at evening, midnight, cockcrow, or dawn. They fail to watch, Jesus finds them sleeping, and they miss “the hour” that very night, when Jesus is arrested.
Mark is saying be prepared at all times for “the hour,” for it will come suddenly; indeed, it came for Jesus the very night he gave the teaching. So sudden it already happened.
This very night.