There doesn't appear to be any logical pattern to the discounts - they are released based on occupency for the same time last year.
I'd suggest looking at the thread at the top of this page - "what's your rate for 2002/2003" and see when other people have gotten discounts. Or, alternatively, pick a few dates that you'd like to travel, and phone to see if they're available. If you're flexible on your travel dates, you should do fine. You'll find though, that while one hotel may offer the discount for a certain week, the other two might not necessarily do the same. So you may need to be completely flexible on your dates if you are adament about where you want to stay.
Welcome to the frustrating world of "dicount seekers"!