Weekends only, and just from 7 July to 1 September that year!

For more recent details, although only these - for 2012 - were available when
checking in mid-Jan 2013 (and again today, Feb 16th), try watching:
_ http://www.snav.it/en/destinations/pontine-island.html

... and also...
http://www.snav.it/en/timetable-and-fares.html

But note that "daily departures" are a thing of the distant past, even during high summer