Hi:
If your depression continues that long, you probably need your meds adjusted. Read somewhere that once a depression takes hold, it depletes the serotonin and can take months for your brain to correct it naturally, thus the need for meds.
When the days start getting longer, we often become manic/hypomanic. There's a theory that BP is partially caused by our internal clockes getting out of wack leading to more depression in the fall/winter and mania/hypomania in the spring and summer.