I'm planning on taking the overnight ferry from Split to Ancona on Saturday Sept 29th. Do I need to reserve now or can I just get a ticket when I'm down there? How far in advance should one reserve if one has to. is two weeks ok?

I'm not sure Jadrolinija take ferry reservations at all, perhaps someone can clarify.
Since you'll be going end of September I'm sure you'll be fine buying your ticket on the day of your journey. The crowds sshould ease somewhat by then.

Jadrolinija certainly do bookings for their overnight ships. They shouldn't be very busy in late September and you'll have no problem getting a ticket for the ship, but may need to book in advance to get a particular type of cabin. I'd suggest booking a few days in advance at a Jadrolinija office.

We took the overnight ferry from Split to Ancona this past June, booking it online about a month before we left since we were bringing a car and also because we wanted to make sure we got a cabin. There were plenty of people who got their tickets the day of the trip, but I agree with GeoffHamer that booking a couple of days before your journey makes sense to give you peace of mind, especially if you want a certain type of accommodation.

Thanks again for your replies. llorac

From what I've read on the internet the international ferries should be booked in advance. I made my reservations about 3 weeks ago for the overnight split-ancona ferry in early sept. and they we already booked for all out-cabins with toilet. I also found Blue (line) ferry was cheaper than jadrolinija. Have a great trip.

By the end of September, there will be fewer people travelling and the ships will be less busy. Blue Line (www.bli-ferry.com) sails most nights in summer from Split to Ancona, but have no sailing on 29 September.