When Happy Days fans feel nostalgic for their favorite Fifties-based sitcom, it's likely that scenes from Arnold's Diner come to mind.

Richie and Fonzie and the gang made it their No. 1 hangout, and through the course of the series, you'll likely remember the diner had two owners. One was named Arnold, as in Arnold's Diner, and portrayed by Pat Morita. The other was named Al Delvecchio, portrayed by Al Molinaro.

It's established on the show that ownership of the diner changed hands during the fourth season premiere in 1976, a three-part episode called "Fonzie Loves Pinky."

From this episode on, we know Big Al as the owner of Arnold's, so many people wrong attribute "Fonzie Loves Pinky" as the first time we saw Molinaro on the show.

In actuality, Big Al appears in the diner before Arnold ever does.

The first episode to feature Al Delvecchio came in the second season. It's called "Guess Who's Coming to Christmas," and it's the first Christmas-themed Happy Days episode. In it, we meet Big Al as a cook at Arnold's, not the owner. That's where the confusion lies, even for people who helped create Happy Days.

During an interview with the Television Academy, even Happy Days screenwriter Lowell Ganz couldn't keep his diner owners straight:

"Al had been there originally," Ganz said confidently. "Al was there first. Al was there before Pat."

But then he loses some confidence, asking the interviewer to confirm whether he remembered that right:

"Wasn't he?" Ganz asked. "Al left, and then Pat came in, and then Al came back. That's what it was. I think!"

Later in the interview, Ganz panicked, wondering if he got it wrong and actually Pat came first, and it was Pat who came back. He was convinced he was right, and the complicated truth is, he was right both times.

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Here's a breakdown of the timeline:

Molinaro appears on the show before Morita, in 1974.

Pat Morita is then memorably introduced in a two-part 1975 episode called "Fearless Fonzarelli." In his first scene, he shoots Fonzie a disapproving look, shaking his head in what would become his trademark move.

Big Al, on the other hand, was known more for simply sighing at the antics of the Happy Days gang.

In 1976, Morita told the Television Academy that he had actually signed on for two or even three more seasons of Happy Days, fully planning to stick around as the diner owner Arnold.

"People ask me, 'Well, why did you leave Happy Days?'" Morita said. "I really didn't leave Happy Days."

He said when the ink was still fresh on his Happy Days contract, he remembered TV producer James Komack (Welcome Back, Kotter) and ABC executive Michael Eisner approached him.

"Pat we think you've got a great future with ABC," they said. "Would you like to have a show of your own?"

"Sure, I'd like that," Morita said, explaining that's what he always wanted for the pinnacle of his career.

But he had one question: "What're you talking about?"

What they were talking about became Mr. T. & Tina, the first series with Morita in the lead.

"So they convince me it would be a right move to make," Morita said. "And of course, it was totally exciting."

Unfortunately, the show ended up totally failing, airing only four episodes. That freed Morita up to return to appear on Happy Days in its 1982–83 season.

During that time, Molinaro continued appearing on the show, with his final appearance in the show's final season.

You can probably see now why the Happy Days screenwriter got lost tracing the characters on the show, and in fact, the first episode to show Al Delvecchio bothered the show writers so much that they actually updated the episode twice to try to keep things straight for fans catching the show in reruns.

So when "Guess Who's Coming to Christmas" aired as a rerun in 1975, it came with a new intro segment where the Fonz is telling Arnold the story of the episode, as if it is a flashback.

Then in 1976, when Morita was gone and Big Al was back, the Fonzie intro segment was reshot with Al instead of Pat, so Fonzie's still telling it as a flashback, but this time to Al.

Most commonly when this episode is aired today, fans see the version with Al. According to Marion Ross (Mrs. C), it's probably one of the few times we saw Al a little more scripted.

"Al Molinaro is another very funny, wonderful guy," Ross told the Television Academy. "The script would just say Al does his thing. And they'd just let him loose."

She said Morita was just as talented with his improvisations, and in his interview, Morita expressed regret over choosing Mr. T. & Tina over more time with Mr. & Mrs. C.

Molinaro appeared in more than 100 episodes of Happy Days and during the 1992 cast reunion, he revealed how much he cherished his time on the show, which he considered a significant piece of TV history.

"In the industry, they used to consider us like a bubble-gum show," Molinaro said. "But I think they overlooked one thing. To the public in America, Happy Days was an important show, and I think it was and I think it still is."