John Calvin Commentary Acts 27:9

"And when much time was spent, and the voyage was now dangerous, because the Fast was now already gone by, Paul admonished them," — Acts 27:9 (ASV)

When sailing was now jeopardous. He does not only mean that the winds were contrary then, but also that the time of the year was not then suitable, which he expresses more plainly afterward, when he says that the fast was past; for I think that this word was added by way of explanation, to note the end of harvest.

Nor do I give weight to the fact that that solemn time of fasting, of which Luke speaks, was unfamiliar to the centurion and the rest of the mariners; for he marks out the times of the year according to the custom of the Jews. Furthermore, we need not doubt that it was the harvest (autumnal) fast.

Although I do not agree with those who think that it was one of the four fasts which the Jews appointed after the exile to Babylon. For Luke would not have simply recorded, without adding any distinction, the third fast, which was in the seventh month, since it was not more famous than the rest, having been commanded to be kept because of the death of Gedaliah, and because of the destruction of the rest of the people.

Again, I cannot tell whether that custom was retained by the people after their return. It is more likely that he means the Day of Atonement, during which the Lord commanded them to humble their souls for seven days. And they began on the tenth day of the seventh month, to which partly September and partly October now correspond (Leviticus 16:29).

Therefore, since they had now entered October, it is said, not without reason, that sailing was hazardous at that time. But if you refer it to hunger (as some do), I do not see what sense can be gathered from that; for they still had a store of wheat in the ship, so that they did not need to be starved by hunger.

And why should he say that the time of the voluntary fast was past? Moreover, it will later appear from the text that they were, therefore, exhorted by Paul to stay because winter was near, whose severity usually closes the seas. For although he was assured that God would govern the ship, yet he would not tempt Him rashly by making too great haste.