John Calvin on Acts 12:4

4. Adding four quaternions of soldiers. Luke doth, in this place, declare by circumstances that Peter was, as it were, shut up in his grave, so that it might seem that he was quite past hope; for as they divided the day and night into four parts by three hours, so Herod divided the watches, that four soldiers might always keep watch, and that one quaternion might succeed another every third hour. He showeth the cause why he was not forthwith put to death, because it had been an heinous offense to put him to death in the Easter holidays; therefore, Herod doth not delay the time as doubtful what to do, but doth only wait for opportunity; yea, he maketh choice of a time, when as his gift may be more plausible, because there came a great multitude together from all parts unto the holy day. “Ad diem festum,” to the feast, or festival.