Charlie Kohlhase slept through September 11, 2001. Not all of it, of course. As well as being one of the most respected bandleaders in the city, Kohlhase is the overnight man at WBUR-FM, running the boards and reading sports and weather during breaks in the BBC broadcasts. He doesn’t get to bed until 6 or 6:30 in the morning. That was the routine on September 11 — except that the phone kept ringing. A little after noon he decided to check his voice mail to see what was going on.

"I’m just getting all these messages like, ‘Well, I’m not sure if the show is on tonight or not, the governor’s declared a state of emergency,’ " he recalls. "And I’m like, ‘What the hell are they talking about? Is there a hurricane coming? No, it’s totally beautiful out.’ "

Eventually he got the word from Regattabar talent booker Fenton Hollander. For a while, no one knew whether Kohlhase and his band — with legendary avant-garde trombonist Roswell Rudd as special guest — would be playing at all, or whether the club would be open. And yet, everyone was in town — Rudd from his home town in upstate New York — and the band had rehearsed the previous night. Meanwhile Hollander’s office kept getting calls; he remembers, "Here we are watching this disaster unfold on television trying to keep our mind on business, which was non-existent, and strangely enough some people said, ‘Is the show still on?’ But nobody called to get their money back. That’s what was weird about it."

Kohlhase says it took a while for the gravity of the situation to sink in. It wasn’t until he drove in for soundcheck at 5 or 5:30 and found no traffic on the roads that he thought, "What on earth is going to happen next?" At soundcheck, the band were in a daze. They’d heard that Rudd was ambivalent about playing, and Kohlhase told his mentor not to sweat it — the quintet would go on, Rudd could get back to his family in New York if that’s what he wanted to do.

Rudd recalls being less sanguine. "To tell you the truth," he says over the phone from his home, "on the morning of the 11th, when I saw what was going on, I really feared for my life and felt that I ought to be home and rounding up my family and just taking care of my own. My first instinct was to put everything in the car and come back to Kerhonkson, New York, ASAP. Because it was in the news already that the planes had taken off from Boston, and so that was enough for me to get out of there." It was the word from Kohlhase’s band that turned him around. "When Charles said, ‘We’re going to play with or without you,’ I said, ‘If you’re going to play, I’m going to play.’ "

"We really weren’t sure we were doing the right thing," says Kohlhase. "But when we actually started playing, with the audience there, there was a certain realness that came into the picture. And on the first tune, Roswell started playing and I felt alive again."

There were a hundred or so people at the show that night, myself among them. At first, Rudd asked the audience to hold its applause. Kohlhase and Rudd, both of whom have an inclination to humor in their work, had agreed to cut some of the more comic elements from the set list. But it wasn’t long before the crowd overruled the no-applause request. Rudd’s music demands participation: it’s fierce, blues-drenched, full of New Orleans-style cross-talk among the horns, driven by an earth-bound songfulness and his own broad, clamorous trombone playing. The show took on a prayer-meeting spontaneity that was at once mournful and celebratory.

Kohlhase and Rudd recall that after their single long set audience members thanked them. "When it was over," says Rudd, "some people came up to me and said, ‘We wouldn’t have known what to do with ourselves tonight if you hadn’t been here.’ . . . But I really have to tell you that I was debilitated by what had happened. It was the hardest gig that I ever played in my life."

This September 11, Rudd and the Kohlhase Quintet (with drummer Eric Rosenthal, bassist John Turner, trumpeter John Carlson, and saxophonist Matt Langley) will return to the Regattabar. Other clubs will be dark that night, including Scullers, and Hollander allows that if it weren’t for Kohlhase and Rudd, the Regattabar probably would be too: "To do anything else would be wrong." And, concedes Kohlhase, with a tired chuckle, "I don’t really know if it’s even a great idea, but I’m just glad to be playing with Roswell again."

As for Rudd, he says the Kohlhase band turned him around that day a year ago. "I went in with trepidation, but as the night progressed, I realized that it was precisely at that time that I should be doing what I was put on earth to do."

The Charlie Kohlhase Quintet with Roswell Rudd plays the Regattabar this Wednesday, September 11. Call (617) 876-7777.