The Tigers are 80 minutes away from Wembley and the chance to win rugby league’s oldest competition for the first time since 1986.
He said: “This is what you play the game for – Sunday will probably be the biggest game of my career.
“I have grown up in Castleford and grew up supporting my hometown club so Sunday will be that extra bit special for me.
“It should be an awesome atmosphere and I would expect to see the majority of Castleford fans there.
“There has been a long time between the quarter-final and the semi-final and Daryl Powell has told us to try and put it to the back of our minds, which has been hard to do but now the tie is nearly here we are just trying to enjoy the week and enjoy the build-up to Sunday.”
Castleford have beaten Widnes in both of the meetings between the two sides in Super League this season.
Clark added: “You can’t take league form into account, or anything else, especially with a place in a Wembley final at stake.
“The game will be different to a league match and it would be massive for both us and Widnes to reach the final. We have not been there for a long time so it will be nice to see a different team in the final.
“Now if we win we’ll be in the final for the first time in 20 years but with Sunday being such an important game, we are not looking ahead of it.”