Awesome to see discussion of hiking outside the popular start dates.
If you're contemplating a late nobo start and you don't want the pressure of getting to Katahdin on time or before cold weather, an often overlooked option to keep in mind is a "leapfrog" thru-hike. After hiking the southern third or half, jump over the hot and humid low elevations of summer in the mid-Atlantic and head up to Massachusetts or Vermont (or somewhere else depending on the timing). That way you'll continue to build your trail legs gradually so you're fully ready for the climbing, ruggedness, and remoteness of New Hampshire, Maine, and Katahdin. Then hike south with fall colors in the Mid-Atlantic.
Thru-hiker registrations by day for 2018 can be viewed this year at
www.atcamp.org (a new system for the voluntary registration system). Although the site has been available for a while, it hasn't actively been promoted yet, so this early there aren't a lot of people registered. I understand charts will be generated a little later this year.
Of course, another thru-hike approach is to start mid-Trail and hike north. If you haven't seen the
Mona Lisa Flip Flop blog post, check it out.
April 15 is on the early side to start from Harpers Ferry (it can be lonely), but it's a pretty good time to start from Rockfish Gap at the south end of Shenandoah National Park. It can still be chilly, but chances of seeing snow, more than flurries, are relatively low. Many of the services in the park are open by then. You could start further south and see more nobos (who will fly past, but at least the provide company, and some will provide helpful tips), but resupply in much of central and southwest Virginia can be more challenging than further south or in the Mid-Atlantic. Pearisburg, VA north to Catawba/Daleville (Roanoke) area is one of the leanest areas for resupply on the whole Trail.
Choosing to hike outside the bubble, if that appeals to you, can have a lot of benefits for the Trail. It helps not only reduce crowding for others, it minimizes campsite expansion, makes work for volunteers more manageable, and helps give hiker-friendly businesses a longer season instead of being overwhelmed for a shorter period of time.
More info about these alternative options can be found
here.