Despite me being on this thread, I never really suffered legitimate depression. However, mood changes between seasons is rather common, just not to the extremity of what you seem to be mentioning. Although it is rather odd, usually people perk up around summer and fall into depression in winter, but in my case it has always been the opposite. That heat never works out well for me.

But regardless, if you determined this to be the cause of your depression, then you can take a few preventative measures (though not necessarily flawless measures) including adjustments in lighting, acquiring the appropriate vitamins (assuming you suffer from winter depression that is), and adjusting indoor temperatures accordingly. Best of luck.