The Hawks bucked the odds in enemy territory on Thursday night to produce a stunning 32-point qualifying final win over Fremantle at a sold-out Optus Stadium.
Foot and shoulder injuries had kept Day out of Hawthorn's past two finals campaigns, with his recurrent navicular issues in particular raising serious doubts about his long-term durability.
But the 25-year-old made the most of his finals debut on Thursday night, racking up 24 disposals and eight clearances to help guide Hawthorn to the 10.12 (72) to 5.10 (40) victory.
Day was in tears after the match, and his emotional hug with coach Sam Mitchell will be one of the endearing images of the boilover.
"It's been a pretty raw two years," Day told Channel 7 while fighting back tears.
"Everyone wrote us off. And to come over and get it done, it's just so much fun. I've dreamt of this as a kid.
"I just wanted to come in, absolutely grateful, and take away all the pressure.
"No expectation. Whatever was going to be, was going to be. It was so much fun."
Fremantle entered the match as hot favourites, but Hawthorn kept the minor premiers to just one goal in the first half in a stunning display of full-team pressure.
It was a tactic Hawthorn players had vowed to unleash leading into the match.
"It was so special. To come over and knock off the top team, it's not easy, especially in this environment," Day said.
"Our big theme was loving your teammate and doing it for your teammate.
"Not thinking about yourself, but trying to make your teammate's day a little bit easier.
"Coming in as underdogs, we felt like we could play with a bit of freedom. Everyone had written us off, and we used that as fuel."
Fifth-year coach Mitchell also looked emotional as he hugged his players after the match.
"It's a big thing for him obviously," Hawthorn forward Jack Ginnivan told Fox Sports.
"He's won four (premierships) as a player, but being a coach and winning one would be a special thing.
"This was a really big game for us. To win this game and get us a home prelim at the 'G, it's in our hands right now."