Roman guards split their night watch up into four three hour segments. 6pm – 9 pm, 9 – Midnight, Midnight- 3 am, 3 am – 6 am.

The end of the third watch of the night was marked by a trumpet blast at 3 am which was nicknamed gallicinium or “the cock crow”.

Jesus references these four watches and the gallicinium himself in Mark 13:35:

Therefore keep watch because you do not know when the owner of the house will come back—whether in the evening, or at midnight, or when the rooster crows, or at dawn.

This detail not only shows the historical awareness of St Mark as he is writing, but because it is an offhand remark–Jesus is mentioning it in passing not as a main preaching point–it affirms again the historical accuracy of the gospels. But there is more.

This detail also helps us with the specific timeline of the events of Jesus’ trial and execution.

We know that the Last Supper took place at night and that the ceremonial meal probably lasted at least  three hours. In Mark’s gospel they go to the Mount of Olives and that is where Jesus predicts that before the cock crows twice Peter will deny him three times.  The prediction of Peter’s denial also takes place on the Mount of Olives in Matthew’s gospel, whereas Luke conflates this detail into the Last Supper account. Missing in Matthew and Luke’s account is the detail that the denial will take place before the cock crows twice.

Since Mark’s gospel is linked with the memories and preaching of Peter it makes sense to take that account as the most accurate.

So what are the two cock crows?

Speculating now, it could be that the first was the gallicinium. The Roman fortress in Jerusalem was at the corner of the temple mount. Therefore the trumpet blast of the gallicinium would have been heard by Jesus and the disciples across the Kidron Valley on the Mount of Olives. Jesus, hearing the gallicinium predicts that before the cock crows again–the second time–which would be the ordinary crowing of a rooster at daybreak–Peter will have denied him three times.

It is the second crowing of the cock which then prompts Peter’s repentance.

This also makes much more sense of Jesus’ reference to keeping watch in Mark 13–the chapter just before he predicts Peter’s denial. The whole chapter is about the end times and the coming persecution. Jesus says right at the end of the chapter,

“But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father. 33 Be on guard! Be alert[e]! You do not know when that time will come. 34 It’s like a man going away: He leaves his house and puts his servants in charge, each with their assigned task, and tells the one at the door to keep watch.

35 “Therefore keep watch because you do not know when the owner of the house will come back—whether in the evening, or at midnight, or when the rooster crows, or at dawn. 36 If he comes suddenly, do not let him find you sleeping.37 What I say to you, I say to everyone: ‘Watch!’”

Then what happens? They let down their guard. They fall asleep in the Garden of Gethsemane.

The gallicinium sounds. Jesus predicts Peter’s denial. The guards come. He is taken. Peter follows to the high priest’s house, denies and then repents.