2 things you can do, depends on whether you willing to spend the time or not.
1) Email the hotels you are interested in to ask when is the earliest you can book for the period you are looking at.
2) Go through each of the hotels website and test water... Check their availability calendar to see how many mths in advance one can book as of today, whether it's 3, 4, 6 or 9 mths in advance. I do this usually because some Japanese website don't provide email address for you to send queries one.
I'm very particular about hotels (cleanliness, smoke free, easy access etc) so I'm willing to invest time to do this because I don't just anyhow settle for any hotel that's available. I stick to my shortlist and my shortlist only. That's why I need the info to know exactly when the hotel is going to release the rooms and I will book it on the day they release.