Albert Barnes Commentary Acts 16:35

"But when it was day, the magistrates sent the sergeants, saying, Let those men go." — Acts 16:35 (ASV)

And when it was day, and so on. It is evident from the narrative that it was not initially contemplated to release them so soon (Acts 16:22–24). However, it is not known what produced this change of purpose in the magistrates. It is probable, though, that they had been brought to reflection by the earthquake, somewhat as the jailer had. Their consciences were likely troubled by the fact that, to please the multitude, they had caused strangers to be beaten and imprisoned without a trial, and contrary to Roman laws.

An earthquake is always suited to alarm the guilty. Among the Romans, it was regarded as an omen of the anger of the gods and was therefore suited to produce agitation and remorse. Their agitation and alarm were shown by the fact that they sent the officers as soon as it was day. The judgments of God are eminently suited to alarm sinners. Two ancient manuscripts offer this reading: "The magistrates, who were alarmed by the earthquake, sent," and so on — (Doddridge). Whether this reading is genuine or not, it doubtless expresses the true reason they sent to release the apostles.

The sergeants. rabdoucouv. Literally, "those having rods"; the lictors. These were public officers who went before magistrates with the emblems of authority. In Rome, they bore the fasces before the senators — that is, a bundle of rods with an axe in its centre, as a symbol of office. They performed a role somewhat similar to that of a beadle in England, or a constable in American courts.