Gail, my experience has been that it is pretty random. There's really no rhyme nor reason to when DL puts out their AP4 rates. They released some AP4 dates for late August and the fall in the beginning of August. Last year, I think they released some Jan-Feb dates in early January. But you can't count on previous years for what they will do in the future. I've tried to plan that way, and it hasn't worked for me. The best thing to do is to check the Disboards (and even other fan sites) regularly because we are usually the first to hear the news and spread the word.
HTH.