“Entertainment Tonight,” ingrained in American pop culture for nearly 40 years, celebrates its historic 10,000th episode Friday (7:30 p.m. on WCBS/Ch. 2).

Not bad for a show that had its doubters when it premiered in September 1981 with hosts Mary Hart and Rob Hendren — and remains TV’s most-watched entertainment newsmagazine over 100,000 segments later.

“When the show first started, it was an entertainment show and everybody said, ‘How are you going to fill a half hour of entertainment?,'” says “ET” host Kevin Frazier (co-host Nancy O’Dell left in August). “Actually, entertainment has become part of American culture. And there are certain standards in the entertainment industry where people feel like ‘ET’ is a part of their day. I think certain things have withstood the test of time, and ‘ET’ has been passed down from generation to generation — like a magical tapestry.”

Friday night’s celebratory episode will include exclusive interviews with Rod Stewart (on his 75th birthday) and Oprah Winfrey (recalling her time with “ET,” dating back to 1985). The show will also receive a Guinness World Records title as “Longest-running entertainment news TV show.”

“You’ll see some of our ‘Best Of’ moments and those unforgettable ‘ET’ moments that are classics and made their way into American pop culture,” says Frazier. “I can’t tell you how many times I’ve been somewhere and somebody says, ‘My God, I used to watch this show every night with my grandma while we ate dinner and Mary Hart would come walking down — and now I sit with my kids and watch you.

“That’s crazy and flattering, because Mary will always be the queen,” he says. “She invented this genre and being able to work with her and to be connected to this show’s beginnings — I feel like I’m connected to Walter Cronkite.”

Hart retired from “ET” in 2011 after 30 years and co-hosts including Bob Goen, Robb Weller and Mark Steines — and, most notably, John Tesh, who was paired with Hart for 10 years (1986-96).

“It was an amazing time to be on a show like that,” says Tesh, 67, who met his wife, actress Connie Sellecca, during his time on “ET.” “The only way to see a movie trailer in those days was either in a movie theater or on ‘ET.’ I used to see the overnight ratings in some markets and it was sometimes 20 million people. It was like ‘Seinfeld.’

“I remember when it was a ‘closet’ show,” he says. “Nobody would admit they watched it. When I got an offer to come audition with Mary, I had never seen it before and people were saying to me, ‘Yeah, you don’t want to watch that.’ It was so far ahead of its time with music and graphics and the speed of the show. [Producer] Paramount spent a lot of money on it.”

Tesh, who came to “ET” from a news background (including a stint in New York on WCBS/Ch. 2), says he was nearly fired early on because of his body language.

“I was trying to come across as tongue-in-cheek and eventually I came across as I didn’t like Mary, and I got called on the carpet,” he says. “I was told, ‘Your body language is terrible on the show. Mary is the franchise. If you want to keep your job, we’re sending you to body-language school.”

Tesh spent three days in Dallas working with a body language expert. “I felt so grateful for Mary,” he says about Hart, with whom he still keeps in touch. “She could have gotten me fired.”

Frazier says one of the keys to “ET’s” ongoing success has been its ability to adapt to changes in culture and lifestyles — particularly how people watch TV in the digital age on so many different platforms.

“It took us a minute to adjust to that,” he says. “When social media came out, the stars said, ‘Oh my god, I have my own voice for all my fans’ but then they came back and realized there’s something special about ‘ET’ and being on our show and about our viewership and our numbers … we’ve kind of dodged and weaved with the times as things have changed — and we’ve integrated those elements into our show and our culture.”