“The Fast” here is the Day of Atonement (Yom Kippur), the great autumn fast of Leviticus 16, used by Luke exactly as a modern writer might say “after Thanksgiving”: as a date. Yom Kippur falls in late September or October, and in the ancient Mediterranean, open-sea sailing was reckoned dangerous from about that point until spring. Luke's note is a Jewish calendar doing secular work: the fast is past, therefore the sailing season is closing, therefore Paul's warning.