Joan Rivers is slated to appear on NBC’s venerable “Tonight” show this evening, the second time the veteran comedienne has graced the late-night mainstay with her presence in just the space of a few weeks.

Before making a cameo on Jimmy Fallon’s first night, however, she hadn’t turned up on the program for 26 years – about “four noses ago,” in her estimation.

“The last time I was on the show, Melissa was in diapers,” Rivers quipped in an interview after taping her appearance for this evening’s episode of “Tonight,” referring to her daughter and longtime co-star. “Now, I’m in diapers.”

Younger viewers may not realize the import of  Joan Rivers turning up on the “Tonight” screen, but there was a time when the comic was an integral part of the workings of the wee-hours institution. Starting in 1983, Rivers was the show’s permanent guest host, substituting regularly whenever Johnny Carson wanted.

The achievement marked a crowning moment in her career, but she was soon exiled from the throne.   She departed the program in 1986 after getting an offer from a then-nascent Fox to host a rival latenight show. Carson was insulted, claiming at the time he had never been informed of the venture. Rivers was, in her own words, “banned, banned, banned” from the show – a condition that would continue under Carson’s successor, Jay Leno.

Her absence from the Leno era ,”wasn’t for lack of asking. I don’t know what his problem was,” she said. “I don’t know, and you want to know something? I don’t care.”

In recent years, Rivers had been making steady visits to Jimmy Fallon’s “Late Night” – seven times during his tenure on that other program. As producers geared up for Fallon’s tenure on “Tonight” to start, Rivers recalled, “they called me up and said, ‘We want you for the first show,’ and it was so great. I’m so happy.”

Her return to the program offers “a great lesson, a great show-business lesson: You keep going, and things turn around and things come back, and it’s an amazing business because of that. I was so banned from late night, but I managed to survive.” She cited her bevy of current projects, including  the fourth season of “Joan & Melissa: Joan Knows Best,” which debuts Saturday; “Fashion Police” on E!; and “In Bed With Joan,” a half-hour “no holds barred” chat show for digital venues.

Many members of her current audience “don’t know who Carson was,” she said, “don’t care who Carson was.”

She said she appreciateswhat Fallon brings to “Tonight.” which she called an “institution.” Fallon, she said, offers “a different show. It’s much hipper” and Fallon “is so in tune with the popular culture.”

Her time on “Tonight” has prompted some reflection:  “It’s really a very good moment for me, a very good moment for me,” said Rivers. “And I’m very appreciative, and very aware of it.”