In my experience, Holland-America and all of the other cruise lines will hold a sale whenever they need to fill up the ship. I've booked 8 months in advance, and I've booked 9 days before sailing, and the cruises closest to the sailing date were those on which I received the best pricing. Of course, I'm retired and live in Florida, so I have certain advantages that many other cruisers don't have.
My advice would be to locate cruise discount agencies (use your web browser, read the Sunday ads, or peruse Frommer's Budget Travel), and sign up for the weekly email from the discounters. You'll be amazed at the number of bargains that are out there!