Speaking as a section hiker, I like having no more than eleven hours of darkness per day when I'm hiking. Otherwise, I go stir crazy with boredom spending a lot of waking hours in the dark in my tent. So March 21 to Sept. 21 are prime hiking time for me and that's a time frame suited to a NOBO thru-hike. A southbound thru-hike starting around July 1 means some mighty long nights in November and December (unless you do a fast hike) toward the end of your thru-hike.