MV used to shut down after the U.S. Labo(u)r Day holiday, the first Monday in September. More recently things tend to stay open until the U.S. Columbus Day holiday, the second Monday in October. That will be the 11th next year. (This creates problems for businesses that depend on student workers who return to school after the summer season, but they find ways to cope.)
The fast ferry schedule for the latter part of next October is shown on their Web site (
http://www.nefastferry.com/schedule.html) as two runs per day. The later one leaves New Bedford at 3:45 pm every day. The early run is at 6:30 on weekdays, too early to be practical for your purposes. On weekends, it's at 9:30.
If your trip is earlier in the month, there are five trips per day.
Of course, any schedules ten months out are subject to change.