When Happy Days star Henry Winkler told Jimmy Kimmel Live! about the day that John Lennon came to set, the Fonzie cool-guy actor admitted he didn’t know how to talk to the Beatles songwriter.

He did notice that Lennon seemed shy, so Winkler figured he better start the conversation.

"I didn’t know how to get into a conversation with John Lennon," Winkler said. "Then I just started talking about his Imagine album that he made."

As Winkler started revealing his fandom, Lennon seemed to come out of his shell.

Soon the pair of pop icons were deep in conversation about another Lennon album, John Lennon/Plastic Ono Band.

"He opened like a flower," Winkler said. "It was amazing."

Pretty much everyone on the Happy Days cast has similarly opened up like a flower about this day when a Beatle visited Happy Days.

But you may not know which episode the show was filming on that fateful day when Lennon appeared on set.

"Guess Who’s Coming to Visit" aired as the fourth episode ever of Happy Days, and it might just be the most Fifties throwback episode to ever air.

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In the episode, Richie sneaks out of the house to watch Fonzie participate in a drag race, driving Ralph Malph’s sick new ride.

Not only does Richie get caught in this episode, confronted when Mr. C shows up at the drag race, too, but before the race can even start, police arrive to break up the whole scene.

It’s a taste of American Graffiti that made the start of Happy Days exciting, and now you know that along with the cool cars and cool-guy racers, John Lennon himself was hanging out when it was filming.

 Out of everyone on the cast, it’s likely that Lennon’s visit meant the most to Winkler.

"Henry was desperate to meet John Lennon," Ron Howard told NME in 2016 when he directed the Beatles found footage movie Eight Days a Week: The Touring Years.

"I had nothing but admiration for John Lennon, but I was never quite as fervent," Howard said. "For Henry, it was practically a religious experience. I just said hello — it was all over in 10 or 15 minutes."

The story goes that Lennon and his son Julian spent the entire day on set of the show.

At some point, the Beatles songwriter posed with the boys, flanked by Ron Howard and Don Most on his right and Anson Williams and Henry Winkler on his left.

Winkler’s not rubbing shoulders with Lennon, but instead, he’s got his hands clasping Julian Lennon’s arms in a sweet hug.

If you look closely at this photo, you’ll know it was taken during the first season of the show, because Fonzie’s not wearing his leather jacket yet.

Instead, he’s wearing a gray jacket, and when you watch  "Guess Who’s Coming to Visit," you can confirm that’s what he had on the whole time Lennon was hanging out.

This episode’s title is extremely fitting for Winkler, who had no idea that Lennon would be coming to visit the set that day.

"All of a sudden, John Lennon just came to visit," Winkler said in 2018, still in awe of this highlight from the earliest days of his acting career.