I thought September/October of 2009 was the lightest I have ever seen in my time.
The busiest? Well, put it this way. I thought I had it all figured out after Thanksgiving in 2017. Hey, why not go to Florida right after Thanksgiving? Everyone is settled back in at work or at school and no one travels then right? Well................wrong. I know the Christmas season used to be light, but not anymore. We went the last week of November and I have never seen it so packed. We just stayed a week and only saw the Magic Kingdom so maybe the other parks weren't so bad, but we did go on a night previous to when the park was to be shut down at about 4:30pm for a staff party and the day AFTER when the park was shut down for Mickey's Christmas party around 6pm. Stupid. Look, we had a fun day, who doesn't at Magic Kingdom? But this is the stuff we missed out on the first time our kids went to Disney World:
Jungle Cruise, Tom Sawyer Island, Peter Pan, Snow White's Mine Train, Thunder Mountain, Space Mountain, Hall of Presidents (closed for refurbishment), Buzz Lightyear, Monsters Inc. Laugh Floor.
No joke, and that was just based on sheer volume of people in the parks. Yes we utilized the fast passes, and still that's what we missed. The last thing we did that day before the fireworks was Haunted Mansion. One of my daughters was so tired she missed the whole thing, she was just buried in my shoulder, sleeping. Missed the fireworks too.
Of course, even missing all of that you still have a great day. We still did a lot of stuff too.