My husband has cyclothymia, a mild form of bi-polar depression. His bad times of the year followed a pattern for many years. He is on medication and uses a light box so that has helped a lot in the past 12 months.
October was always a really rough month for us too, which really stinks because our anniversary is in October. For him I think the change of seasons is a trigger - he really, really needs sunlight to function and getting up when it's barely light out and having it be dark by dinner time are hard on him. The light box has helped a lot with this. In our marriage, the bad cycles would last about 6 weeks so normally he was better by Thanksgiving and could hold it together through the December holidays, which was always a relief.
Our other predictable bad time of year was February - almost all of our Valentine's days have been a disaster. This has sometimes spilled over into March and April, making for very tense spring holidays (Easter, Passover and 2 of our kids birthdays).
Things are definitely getting better over the years - during the worst year, I spent 3 weekends in a row outside with 4 small kids trying to do yard clean up and rake leaves while my husband spent those days in bed with the shades down. Other than going to work and picking up the kids, he stayed in bed and totally ignored all of us for almost a month.
You're definitely not the only one!