2007 is a long way off right now to make specific plans.
For example, the current schedule for Fall of 2005 doesn't show ANY 4 day trips. They currently only show 7 day trips. And interestingly enough, they leave the same day (one Eastern, the other Western).
By 2007, there could be one or more additional ships, and you don't know what kinds of iteneraries will be available.
Sept and Oct seem to be the cheapest times. If someone wanted to book a cruise for this year, the cheapest time seems to be any week in Sept, with Oct costing a few hundred more.
As for when to book, go ahead an book as soon as the itenireries for Fall 2007 are released. Based on this year, it sounds like you'll have to wait until around January 2006.
[Edit]
I'm mistaken.
DCL does show 3/4 night cruises in Fall of 2005. It appears that they have a mistake (one that has existed for about a month now) on their page where they list special pricing for future dates. For some reason, that page shows Eastern and Western 7 day cruises sailing on the same dates (for example, they show a 7 day Eastern and a 7 day Western sailing on Oct 15).