During fall and winter months, our air conditioners can sit idle. This can cause water in the drain line to gunk up and cause a clog. I have even heard of A/C drain lines becoming clogged due to different types of nests being built in the drain line.
Remember to get your heaters checked before the winter months and get your air conditioners checked before the summer months, especially the drain lines.
If you are thinking of getting any duct work done, fall is the best time to get it done, because our attics stay much cooler. This is good for your A/C tech and good for you. Because a lot of that hot attic air is going to end up warming up your home as well. I think some A/C companies charge less for ductwork in fall and winter months.