NEW YORK -- It's a holly, jolly Christmas for Saturday Night Live chief Lorne Michaels as he marks another holiday edition of the show he created 36 years ago, and as he welcomes back SNL alum Jimmy Fallon as guest host for this special yuletide bash.

Boasting singer Michael Bublè as its musical guest, the program airs, of course, at 11:30 p.m. Saturday on NBC.

But on Thursday night, as Michaels welcomed a reporter to his Rockefeller Center office -- overlooking Studio 8H, from where SNL originates -- the clock was ticking: little more than two days until show time.

In the studio, a sketch was being blocked for the cameras: Denver Broncos quarterback (and famously devout Christian) Tim Tebow confronts Jesus in the locker room.

"It's been rewritten since last night when we read it," Michaels said. "We read 40-some-odd sketches yesterday, and narrowed them down to the pieces over there" -- he pointed to a board with a tentative rundown -- "and that's about 15 minutes [too] long. By the time [Weekend] Update gets done, the show will be maybe 25 minutes long, which is what we'll go into dress rehearsal with."

Dress rehearsal takes place in front of a live audience on Saturday evening, after which, according to the audience's response, the show is rearranged, cut, and otherwise revamped during a crash session to whip it into shape to perform a couple hours later for the world.

For Michaels, dress brings pain every week.

"Things you were certain would work, don't," he said. "Things that were really bright flatten and fall apart."

So does this mean that, even after all these years of executive-producing SNL, Michaels, the old hand at 67, is still caught by surprise at how an audience reacts?

"Every week," he said. "I think it's why I'm still here. It's not a thing you ever master."

Or do you?

"Lorne has done this for 36 years, and he knows what will work," Fallon said in an another interview. "He's a pro. He's a Beatle."

Fallon was an SNL cast member for six seasons before leaving in 2004. Then, in 2009, he was tapped by Michaels, who also executive-produces NBC's Late Night, to fill its hosting job when Conan O'Brien graduated to The Tonight Show.

Now, for the first time, Fallon has been invited back by Michaels to his old haunts at SNL to serve as host.

"There will be holiday-themed sketches for different religions," said Fallon, who said he arrived with "about 300 ideas" on Monday.

"I'm working on some impressions that I haven't done before. I've got some surprises: Some old friends might be coming back for a cameo or two. And then I want to see if I can dust off my Update suit."

Fallon was asked if any of the special demands of SNL had been hard to face again.

"Staying up late," he said. "I don't do that anymore. I have a 9-to-5 job now with Late Night. I got to work on keeping my energy up, so I'm ready to go on Saturday. Which I will be."

Once he steps onstage at the top of the show, "it's going to be an adrenaline rush," he predicted. It will also be an emotional rush to be back, in a proud guest-host role on the show he has loved all his life.

"I just hope I don't break down and cry," he said. "My mom and dad are going to be there. I got to make sure I don't make eye contact with them. I'd be a mess, a blubbering mess."

First Published December 17, 2011, 5:00 a.m.