Alien Season: When Is It?

Now that it's getting colder (at least here in the midwest), I'm wondering if the chances to see a UFO or an alien running around through the woods would be more, less or about the same as any other time of year. Somehow I think they like the hotter weather, but I couldnt say for sure. Anyone ever research to see if there are particular times of year that aliens tend to come out more than others?