Toward the end of summer tends to be one of the best times to have your AC system serviced. Why is that? First, you can still run it. After it gets below 65 degree you can actually break your AC unit. If you call the HVAC guy in January, it may be too cold to run it and he may not be able to service it well.
Also, the HVAC companies tend to be not as busy this time of year, so you have a better chance of getting them in and having their full attention.