Fall has halloween and and leads right into ney years, summer has…
…4th of July and the bright summer sun?
Fall. The sound in an indoor stadium is almost always louder plus you can do better lighting effects and such. And, of course, you can always get rained on outside as well…(though the 7/23/99 show when it POURED was just plain AWESOME…the rain storm gives Phish jamming powers…a badass Free and a 30-minute Birds? Thank you, may I have another!)
Stevo
As far as vibe, I enjoyed Summer. There was nothing like strolling the lots during a warm early evening browsing the vending, (I always brought my own ‘refreshments’ so that wasn’t on the agenda), buying some shirts, stickers, etc. I loved cooking out, selling some tasty cheesburgers for $1.00 and soda for 75 cents. Didn’t make any money, but a cheap meal always got a smile and a “thanks” in return, which, for me, was well worth it. But as far as music, Fall tour, no doubt. The roar inside the auditorium as the houselights went down was deafening, and I LOVED IT!!! 
Plus Kuroda’s lights were something to behold.
I’ll take any Phish any day of the year anywhere.
Most of the shows I’ve been to were in the Summer. Those were awesome. I’ve been to four shows in the Fall. Those were awesome too. I think I’m seeing a trend here. I’ll have a summary with visual data graph things on your desk by 7:30 PM Vegas time.
If Fall means indoors, then I’m going with Fall. I’ve been to too many shows that were either rainy (All of Northerly 2013 and Deer Creek 2016) or scorching (Toyota 2009, Tuscaloosa (98 F at showtime!) & Nashville 2015, Dick’s 2017). The weather was gross all weekend and it didn’t affect the Rosemont shows in the slightest.