Difficult to pin down the best day, but one sure comes to mind...
My family was never the outdoors type, and I grew up in the South, but as I kid I always longed for the mountains and the West.
My first vacation on my own, after I'd moved away from home, was to Yellowstone National Park. I'd never been to a national park before, nor any of the Rocky Mountain states. I flew into Billings, Montana, whose airport sits on top of a bluff. The arrival was awe inspiring.
I rented my car, a little 3-cylinder Daihatsu Charade. I'll never forget that. A little 3-cylinder car trying to make it over Montana mountains. I had it floored, and it was barely moving.
As I drove west from Billings to Bozeman on I-90, it's like the whole world opened up. Words can't describe it. I could see why they call Montana "Big Sky Country". The open land, blue sky, clouds, the mountains in the distance. That drive, and visiting the park later, literally changed my life.
I moved to Denver sight unseen a few months later because of that trip, which started my love affair with the Rocky Mountain west. It's one of the reasons I want to so desperately move back there for at least a couple years when I quit in about another year.
There's a bit of my soul I left in Montana, Wyoming, and Colorado when I left, and I need to go get it back and do those trips again.