Booking for Carnival to Hawaii is approximately 12 months ahead of time, so you'll be waiting for a while to book it.
But use an agent and let them take care of it for you instead of you hassling with it. That's their job and it costs you nothing for their services. Plus, if you have any problems, they represent your best interest. If you book it with the cruise line and have problems, who do you thing they'll represent?
Also, since this is such a big occasion, why not consider something nicer than the Miracle? There are definitely some better ones making the trip. Having been on the Miracle, personally I wouldn't want to spend more than a week on it - just not that big and not alot of amenities.
While it would cost more, you could also think about doing the 7-night cruise on the Pride of America. If you really want to see the islands, you can't beat their itinerary. We did this along with a week before in Oahu and a week after in Maui. Awesome romantic adventure. But then we were taking the kids.
Talk to you agent, preferably one who is a cruise specialist, has been there done that, and can offer some options and ideas for you to make this a wonderful anniversary.