John Calvin Commentary Acts 12:4

"And when he had taken him, he put him in prison, and delivered him to four quaternions of soldiers to guard him; intending after the Passover to bring him forth to the people." — Acts 12:4 (ASV)

Adding four quaternions of soldiers. Luke, in this passage, makes it evident through the circumstances that Peter was, as it were, shut up in his grave, so that it might seem he was entirely beyond hope. For, just as the day and night were divided into four parts of three hours each, so Herod divided the watches, so that four soldiers would always keep watch, and one quaternion would succeed another every third hour.

He shows the reason why Peter was not immediately put to death: because it was a heinous offense to put him to death during the Easter holidays. Therefore, Herod does not delay as if doubtful what to do, but only waits for an opportunity. Indeed, he chooses a time when his gift might be more applauded, because a great multitude came together from all parts for the holy day.