Does anyone else get one at the exact same time every year, like clockwork?

Mine shows up the first week of September almost every year, and I have no idea why, it is not even that hot or cold here yet. I started thinking maybe it is the back to work stress or the change in light, but honestly it feels like my body just has it on a calendar. Does anyone else have a season or a month that is basically guaranteed for them, and did you ever figure out what the actual trigger was?