Huddersfield Town's commercial director Sean Jarvis is urging fans to back the Terriers as they face Sheffield Wednesday in "the biggest game in the recent history" for Town.
David Wagner's men face the Owls at the John Smith's Stadium this weekend in the first leg of the Championship play-off semi-final, with the aggregate victor booking themselves a place at Wembley on May 29.
And Jarvis believes 'Wagner's barmy army' can make all the difference in the home tie this weekend.
He said: "Sunday is probably the biggest game in the recent history of this football club.
"45 years we've been waiting for this kind of fixture so the fans I think play a truly massive part in the game on Sunday.
"They've been phenomenal all season - to be averaging 20,000 in the second tier of football which we've not had for some 75 years is amazing.
"This place is the place to be.
"For me, the colour and the atmosphere they've brought this season has been second to none.
"I think a lot of clubs have been envious of what has happened here.
"I remember the first game one of the opposite fans came up to me and said 'you've got something special here' - and I think that's been true throughout the season.
"So for Sunday it is huge, it is massive. I don't say those words lightly.
"The fanbase have to bring everything they've got.
"We've used a phrase this year - leave nothing behind. I guess I'm asking for the fanbase to do that.
"Wear something blue, be loud, be proud, sing, shout and get behind the lads because they can make the difference.
"If they turn up as they have been doing all season then they truly will add something to the game."
After an exceptional season, Town's form has dipped in recent weeks, with the Terriers ending the campaign with one win in their last five outings.
But the commercial director is adamant the play-offs are a completely different competition to the league campaign.
"The play-offs is ground zero - we're back to that starting point again," he said.
"The play-offs is a separate competition and as David [Wagner] has been saying it's effectively two games - home and away - first and foremost.
"Here on Sunday so they've got to give it everything they've got.
"They'll dig deep.
"We've heard phrases out there: 'beware the underdog' and 'Terrier spirit' and that's what's got to come on Sunday.
"We're looking forward to seeing that.
"Come Sunday it will be special. It will be a great occasion at the stadium."
There are still some tickets left for the home leg and Jarvis hopes the fans will snap these up when they go on general sale later this week.
"They go on general sale online at 6pm tonight so people can go to the website to see all the information," he said.
"And tomorrow morning from 9am they will be on general sale [at the ticket office].
"At this moment in time I think there are about 6,000 left.
"Behind the goals is going to be exciting - I know the South Stand is sold out already.
"There are pockets around the stadium that need filling and we're urging fans to do that."
Fans will need to have a previous booking history with Town to buy tickets.