Don’t expect anyone to jump out of a cake Friday night on the “Late Show with David Letterman.” Don’t expect much of a celebration at all, in fact. But what is so special in the life of 54-year-old Letterman on Feb 1? The date marks the 20th anniversary of Letterman’s late-night debut, as host of “Late Night” on NBC. (He jumped ship to CBS in 1993.) Mathematically speaking, it will be Letterman’s 3,558th broadcast. “I don’t think it’s going to be a big celebration,” executive producer Rob Burnett told New York’s Daily News. “We decided to low-key this one.” Regis Philbin and comic Steven Wright are booked as guests Friday night, and as noted by the Associated Press, having Reege around is something special, of sorts. When Dave broke the news on his show two years ago that he was facing heart surgery, it was with Philbin in the guest’s chair, and the “Who Wants to Be a Millionaire Host,” 68, was also there for Letterman’s first day back following his recovery. Philbin was also the first person Letterman quipped with on-air during his first show after the Sept. 11 terrorist attacks. (Friday will mark Philbin’s 61st appearance on the “Late Show,” second only to Tony Randall’s 70.) So, TV observers are predicting that Letterman may mention that he’s marking his 20th, but not do much beyond that. Something else to celebrate: Though his NBC rival, Jay Leno, 51, still leads the ratings battle in their timeslot, Leno had 6.3 million viewers a year ago, but now only has 6 million. Dave’s ratings haven’t slipped at all — 4.36 million this year, up slightly from last year’s 4.33 million. But he’s also increased his viewership among the younger audience, aged 18-49, by some 12 percent, according to the Nielsens.