I will let you in on a secret: no one, not even the big publishers, know exactly when to publish a book. Yes, there are some givens, like making sure you are able to get into holiday and other promotions like Christmas, Mother’s Day, Father’s Day, etc. Then there are books by authors that consumers are trained to buy in a certain month based on its availability. I’m talking about Michael Connelly, James Patterson, and others who write at least one book per year.
I should also mention the reason a book is usually published on a certain date is because of marketing and publicity reasons. We try to get books out there when they will be featured most prominently and when the media are interested in what the titles and authors have to say.