As with most travel and accommodation options, the further in advance you book, the more likely you are to get good deals and cheap ferry tickets.
Booking in advance also makes it easier to get the exact dates you need, to correspond with your holiday accommodation booking. This is especially important if you have specific needs, like a family cabin or a pet-friendly cabin. So, we recommend booking your campsite and your ferry travel at the same time.
Also, be aware that prices will generally be higher on more popular sailing times and dates. Sailing during the day on longer crossings, booking weekday trips, and avoiding travelling around the school and public holidays (if possible) can save you money.