I live in NJ not too far from the Delaware Water Gap. I would like to start my thru hike at the DWG and head south to Springer Mtn. and the come back to the DWG. For the second part of my thru hike I would like to start at the DWG and head north to Katahdin and then back to the DWG.
I would essentially be doing the entire trail twice. My question is:
What would be the best time of year to start at the DWG, head north to Katahdin and come back?
What would be the best time of year to start at the DWG, head south to Springer and come back?
Would I be able to complete this in the same year? If not, that's ok.
It's 895.1 miles to Katahdin from Delaware Water Gap. I think you should be at the northern terminus about September 1. When you start depends upon how long you will take to cover that distance.
I'll start with the north end of the trail since that's where you'll have some time limitations. The Green Mountain Club, which supervises the Long Trail, says that during mud season, April 15 to Memorial Day, "hikers are urged to stay off hiking trails," although some hikers ignore this and hike anyway.
Baxter State Park does not open up the trails to Katahdin until around June 1 each year, depending on weather, and the park's camping areas close after Oct. 15. You can still summit Katahdin after this, but you must pick a good weather day and start your day from outside of the park. To be on the safe side it's a good idea to summit Katahdin by early October at the latest.
Further, this June to early October window is a good one to keep in mind when going over Mount Washington on the way both north and south. According to the Thru-Hiker's Companion the building at the top of Mount Washington operated by the New Hampshire Park Service is only open from May to early October, weather permitting.
So, all of these things put together mean that you can make the case that a responsible hiker would not want to arrive at the Vermont line going north before June and would want to be beyond the top of Mount Washington going back south no later than early October because of potentially dangerous weather, and preferably earlier than that. Since a "typical" thru-hiker would most likely take two months or more, depending on the hiker, to hike this 850-900 mile stretch, it would be a good idea to arrive at the Vermont line going north in June or July.
And you wouldn't reach the Vermont line until you had already hiked about 2850 miles from DWG to Springer and then back to Vermont. You would need to have a decent idea of your future hiking speed before you set out to make this work.
Can you do that? If not, you might be better off doing the northern round trip first, and then doing the southern one. Either way, unless you are a pretty fast hiker you had better enjoy winter hiking and camping because you're likely to have some!
Now as I read your original post more closely I see you are willing to break the northern and southern round trips into different hikes if you had to, and that would make your task less daunting than I'm making it sound. Your question involves some variables like hiking speed and whether you prefer hot weather or cold weather hiking (though with your screen name I'm guessing winter might be all right with you). So when you would want to do the southern loop is hard to know. But the northern loop involves those restrictions I mentioned, so a good window to be leaving DWG for that part would be mid-May through June I would think. That should put you at the Vermont border in either June or July.