My 2 cents is go based on when its most convenient for your family, not when you "think" its gonna be least crowded. Everyone is obsessed with finding "the best" time to go... its different for everyone... here's why:
At "slow" times they cut hours, refurb rides, and reduce capacity making lines longer and everything "feels" more crowded. Also people read everything on the internet before their trips, so its not necessarily low crowds anymore. Homeschool families and families who pull their kids outta school go during this time because "the internet said it wasnt crowded" so they expect the closed deserted theme park from Griswald's family vacation and complain when there are other people at Disney. (January, February, September, Early Dec)
If you hate the heat avoid June-September... We are southerners and everytime we have gone in the summer, it feels like home (HOT) but we are at Disney so Heat be darned... Epcot is the least crowded in summer when there are no festivals...its empty. The other have very moderate crowds. Of all the times of year we have visited, I feel like summer is the new low season. People show up at 9 or 10 and "flame out" by mid afternoon. If you are a southerner go in the summer and watch those Northerners evaporate after the 3PM parade. Or... TAKE A BREAK and come back in the evening.
If you want to go during Spring Break, DO IT! Weather is mild, and you are on spring break flower and Garden is Awesome at Epcot but will be more crowded, luckily most Flower and Garden attractions dont require lines (except for the food booths around lunch and dinner) I think this is the best Epcot Festival for kids. They love looking at the flowers, topiaries, and butterfly house, and you dont need to wait in lines. The weather is milder too so they don't overheat.
If you want to do Holidays, Great! Christmas is magical The parks have very long hours. They are open very early and very late. Its
crowded, but the word is out that the holidays are crowded so they seem to be a little less so. We have gotten the most done ever at Disney on Dec 23rd. The park opened at 7AM. It was empty. We rode 15-16 rides in 4.5 hours then watched the Christmas parade (no party ticket required) and ate lunch (longest line of the day) before leaving around 1PM.
If you can go in the fall, great! Crowds fluctuate based on school holidays and various events. Food and Wine at Epcot will be slammed on weekends and very... Sloppy. Not just at night but it gets worse the later it is. Lots of Drinkers, Bachelor/ Bachelorette parties, drink around the world groups. Definitely an adult atmosphere. Kids are always welcome, but I would head home early or switch parks after lunchtime unless you like watching drunks/ people leave food trash everywhere. (Also Summer in the South lasts until Oct-Nov so lots of burned/ dehydrated folks at Epcot) Weekdays are often less crowded property wide unless there is an event or a particular holiday.
The point is, have a smart plan, arrive early and be flexible. Disney has learned that cutting back hours saves money and lets them sell an extra set of tickets for after hours events (no more Freebie 2 AM closings of yesteryear). Those days in the early 2000's after 9/11 or early 2010's after the financial collapse when you could just roll up and do whatever are gone (for now). People remember only their good Disney trips of the past on these boards but forget all of the rough stuff going on in the real world because we got thru it. The Coronovirus has the chance to be the next thing that scares people away from the parks. Embrace the wild, wonderful craziness that is WDW.
