Ellen Degeneres and Portia De Rossi
Ellen DeGeneres and Portia de Rossi Christopher Polk/Getty Images for People's Choice Awards

"I'm Ellen and I'm gay." This is how Ellen DeGeneres starts her upcoming special episode of The Ellen DeGeneres Show to celebrate the 20th anniversary of her coming out on TV. On 30 April 1997, she created television history when her character Ellen came out as a lesbian in a two-part special titled The Puppy Episode on her sitcom.

"Twenty years ago I said that and it was a much bigger deal then," DeGeneres recalls in her opening monologue as she paid tribute to the iconic TV moment. "This was before Facebook, so if you wanted to announce something, you had to get a sitcom. And because 'Ellen Throws Her Career Away' seemed too on the nose," she added.

"I can't describe to you how challenging it was to get this episode made. At the time, it was so controversial," she said. "Which is why I thought this was important to celebrate the anniversary and to remember what it was like back then and to appreciate how far we've come."

The episode, which will air on Friday 28 April, has Oprah Winfrey and Laura Dern as special guests to commemorate the milestone. In the 1997 sitcom, Dern's character helps Ellen Morgan realise her sexuality. DeGeneres' dual announcement (once on 14 April 1997, on the cover of Time magazine and then on the sitcom) took the TV world by storm but it later paved the way for more same-sex oriented roles.

Winfrey, who played a therapist on the historic coming out episode, lauds Ellen for her courage to risk her career and set an example for her followers. "You would not be who you are ... you wouldn't be able to open hearts and touch hearts and change peoples' minds and make a difference in the world had you not had the courage to do that. And 20 years ago, you had no idea it would put you in this seat."

On the struggles that homosexual people face, DeGeneres said: "We're not supposed to confirm. We're not supposed to be like somebody else. We're not supposed to act like somebody else. And as long as you stay true to exactly who you are, you will be rewarded in ways you can't even imagine."

"For you straight people out there, gay people just don't go around and say 'I'm gay,' because you don't need to. For the people that know you're gay, they just know when you say it. For those words to come out of my mouth for the very first time to say 'I am gay,' even rehearsing it... I would burst into tear every single time," DeGeneres said in the preview clip.

"I have to say watching the clip made me so emotional," Dern said, reacting to the special scene she shot with Ellen. "[There is] no greater gift than being the person that was with you and looking in your eyes as you said those words," Dern told the audiences as she recalled the rehearsal moments when DeGeneres whispered 'I am gay' in her ears. "Maybe I'm not going to say it, because I haven't said that out loud."

"Watching you have this catharsis and the audience support, literally holding each other up during this very emotional moment, it was so profound," the Big Little Lies actress added.

The special clip concludes with DeGeneres kissing her wife of nine years, Portia de Rossi. The special episode of the Ellen DeGeneres Show will air on Friday on WNBC network.