I get situation depressions. Eg: My Birthday, Christmas, break-ups. Those ALWAYS lead to a situation depression. But they might just last a few weeks - sometimes shorter sometimes longer.

My actual depressions show up whenever the hell they want to. Sometimes I'll be in one before my typical situation-triggers, so they just get absorbed in the bigger depression.

Sometimes if I am stuck in too many situational things in a row (typically this means that I'm stressed out and not handling things well) then it can push me into a depression that will last a lot longer - I consider that a BP/true depression as it will last way longer than the situations that prompted it (as in, I won't be upset about those particular events anymore... but the depression is still there).

If there's a BP/non-situational depression, I won't be able to pin-point when it started, and can't think of causes for them. They're just there.