Albert Barnes on Acts 13:44

And the next sabbath day. This was the regular day for worship, and it was natural that a greater multitude should convene on that day than on the other days of the week.

Came almost the whole city. Whether this was in the synagogue is not affirmed; but it is probable that that was the place where the multitude convened. The news of the presence of the apostles, and of their doctrines, had been circulated doubtless by the Gentiles who had heard them, and curiosity attracted the multitude to hear them. Comp. Acts 13:7.