I Just read through this thread again, and thought about it more...
I think we might be going in the wrong direction.
I don't think looking at what quarter major label artists release material would be helpful to people of our level.
For instance, Maybe most of Jay-Z's album sales came through BestBuy,
His label would look at BestBuys highest selling season for rap music, and factor that in to when they drop his album.
Non (well most) of us dont have our albums in BestBuy, so our info would be skewed. I think we should look at this with a more direct appraoch.
For instance...
If most of your album sales come through online distributors, I think Winter would be the best time to promote the album. Because more people stay indoors during the winter and do things like, brows the internet.
If you have a college campus in your city, Fall would be a good time because the students are just returning and arnt dreading being there yet.
etc etc...
I dont think theres any 'magic' time. I think it's best if you factor in all the variables in YOUR situation, and brainstorm times you would be most likely to get people to buy your album.