That would really depend on the cause, certainly hurricanes would also happen around the same season every year. Other possible SHTF disasters would have little or no effect on the time of year in which they occur. They would be just as deadly in the fall as in the peak of winter.
If a disaster were to occur in the dead of winter or at the peak of summer, there effect would be multiplied. Many people use electricity to heat and cool their homes, many people would freeze to death without a way to stay warm.
So many people have become adapted to living in air conditioned buildings, they actually struggle to survive without it. They no longer possess the knowledge or ability to deal with severe heat.
- Plague or Virus is another attack which could be made much worse, based on the time of year it occurs.
- Without power, water will become an issue in the heat much faster, as many water sources will dry up quickly.
- Many people use electricity to heat and cool their homes and in the dead of winter, many people would quickly freeze to death without a way to stay warm.
People move around less in the winter and are much easier to track when they do. Combine this with the other effects of a disaster occurring and you have a nightmare scenario.
It would be the type of virus or plague that would determine the time of year which it would be the most destructive.