A scene from "The Day of the Doctor"

David Tennant, right, as the Tenth Doctor and Matt Smith as the Eleventh Doctor in "The Day of the Doctor," marking 50 years of "Doctor Who." (Adrian Rogers / BBC Worldwide)

"The Day of the Doctor" (BBC America, Saturday), "The Night of the Doctor" and "The Last Day" (online, at your will), "An Adventure in Space and Time" (BBC America, Friday). "Doctor Who," the television show/British national monument about an alien gadabout who travels all of creation, from end to end and first to last, in a living blue police call box, turns 50 Saturday. It will be marked properly, not with a look back (though there have been those as well) but with an actual, brand new, extremely special episode, "The Day of the Doctor," whose particulars are being kept very secret, except for the bits that aren't. What we do know is that it will feature, dream-teamed, the current occupant of the part Matt Smith (the Eleventh Doctor), and his predecessor David Tennant (Doctor 10). For the uninitiated, the character regenerates every so often, taking on the shape of a new actor -- but every fresh Doctor remains identical with all his previous selves. It's not like Sean Connery and Roger Moore playing James Bond one after the other; rather, it's as if James Bond looked and acted like Sean Connery one moment and, after a brief interlude of special effects, looked and acted like Roger Moore the next. (And then the Connery Bond and the Moore had an adventure together, side by side.) We know as well that Billie Piper will reprise her role as Tenth-Doctor companion Rose Tyler, and that John Hurt will be in it, also as the Doctor, in some way yet to be explained. (Let's call him Doctor Too.) Surprises are in store, one assumes.

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Some hints are contained in the online prequels "The Last Day," which seems to be a scene from the Time Wars (POV, but whose?), and "The Night of the Doctor," in which Eighth Doctor Paul McGann, who had previously played the role for the space of a single TV movie (and myriad radio dramas), gets a long-awaited second screen appearance. (That means we have jumped back, timeline-wise, or timeywimeyline-wise, into the space before the series was revived, in 2005, after its 1989 cancellation -- but I see I am confusing you.) It probably won't be necessary to watch "Night" in order to comprehend "Day," but watch it anyway. If McGann (more lately of "Luther") hadn't already been the Doctor, and if the position hadn't already been filled -- Peter Capaldi is soon to replace Smith -- he'd make a great next one.

Also preparatory to the big event is "An Adventure in Space and Time,” a new TV movie (written by sometime "Who" scripter Mark Gatiss) about the beginnings of the series -- an origin story, as they say in the comic books -- with David Bradley, of "Broadchurch" and "Blackpool" and Harry Potter films as First Doctor William Hartnell. (He has the forehead for the part.) It's a tale of old changing times, as an unlikely trio (flashy Canadian head of drama Sydney Newman, played by Brian Cox; first female BBC producer Verity Lambert, played with sharp-elbowed elegance by Jessica Raine; and first Indian director Waris Hussein, played by Sacha Dhawan) mounts a low-budget science-fiction show that gets little respect until 10 million people tune in. (Thank you, the Daleks.) Unlike Hartnell, whose failing health led him to leave the series at 58 (thus triggering the regeneration scenario, as Patrick Troughton became the Second Doctor), Bradley, a vigorous 71, seems to be everywhere, all the time. A superstar among character actors, he's the right man for this job. The forehead is just a bonus.

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"Carol Burnett: The Mark Twain Prize" (CBS, Sunday), "Bill Cosby: Far From Finished" (Comedy Central, Saturday), "Sarah Silverman: We Are Miracles" (HBO, Saturday). Tragedy grabs the headlines, but comedy has as much to tell you about who you are and how things are; it cuts deep, but it leaves you laughing. It puts things in perspective and in proportion. It can make you a better person (or, to be fair, a worse one, depending on the comic).

Carol Burnett received the Mark Twain Prize for American Humor last month -- "an award that Mark Twain himself still has never won,"says Tina Fey, who was awarded it in 2010 and is the first to speak in the TV broadcast of that ceremonial tribute. After Lucille Ball, Burnett is perhaps television’s greatest comedienne, if only in terms of the reach of her influence and the warm regard of her fans. Her metier was sketch comedy, and as such she's a godmother to "Saturday Night Live" alumnae Fey, Amy Poehler and Maya Rudolph, all of who appear here; Poehler, in a gray wig, baggy dress and misbuttoned sweater, came out leading a pack of dogs as "Roz Dozer," Burnett's "longtime personal assistant"). ("I usually stay beside the scenes, but Carol said, 'No, Roz, I want you up there -- I want you to stand onstage and publicly apologize for getting my coffee order wrong." Burnett's famous ear tug, she said, was not a message to her grandmother, but "actually a secret signal to me to go find hot young men in the audience and bring them back to her dressing room.") The two-hour show --  which also includes toasts from Martin Short, Julie Andrews and former castmates Tim Conway and Vicki Lawrence, and a song from Tony Bennett, who turned 87 when you weren't looking and still does his thing without strain -- is often hilarious, and is at least sweet when it stumbles. (I found myself getting choked up even when nothing especially sentimental was being said.) Classic clips include the famous-to-fans "Gone With the Wind" and "Sunset Boulevard" parodies, clips from "The Garry Moore Show" and "Ed Sullivan" (singing “I Made a Fool of Myself over John Foster Dulles”), and "I'm Shy" from the telecast of "Once Upon a Mattress," which made her a Broadway star at 26.

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Bill Cosby, 76 and also a Mark Twain Prize winner (in 2009), has a concert special this weekend, his first such TV show in 30 years, though he hasn't been exactly out of sight in the meantime. (Lately, he has become a favored guest on Jimmy Fallon's "Late Night.") That he is effecting this return via Comedy Central, home of "Workaholics" and "Drunk History" and "Inside Amy Schumer" and many stand-up hours featuring comedians less polite than himself is something he has a little comic sport with at the top. It is a sit-down, rather than a stand-up, performance, but it doesn't lack for energy or physicality -- though one regards him warily at first, for signs of exhaustion or rambling old-guy grumpiness. Cosby is certainly capable of that. He begins talking about men and women and what the latter demand from the former, and analogizes them as chess pieces, the Queen who goes wherever she wants, the King who advances step by fearful step. This is, of course, the stuff of many younger comics' routines, which doesn't make it any more encouraging. But he turns it around; eventually see that he's the fool here, the boy-man who just wants to sneak some cookies -- the subject of one masterfully structured and beautifully performed story. In a climactic piece, driving his kids to school, he melts before your eyes into the younger father he was. There are also familiar dialogues between the brain and the body (gold) and a bit of hand-in-the-Jello-bowl mugging (not so gold), but all in all, the special lives up to its name. Neither finished, nor diminished.

At 42, with many notches in her comedy gun, Sarah Silverman will also be regarded, by freshmen comics, as an elder stateswoman, an inspiration, a role model, or possibly a person to be gotten out of the way so one's own career might flourish. She also has a performance special this weekend, "We Are Miracles," on HBO, filmed at the small bar at Largo, with an opening in which, waiting on the street to go on, she engages with some savvy cholos. ("Largo," one says, "that's like barely 300 seats." "Well, actually I'm doing it in the littler room." "The little room -- that's like 50 seats." "Thirty-nine with the fire marshal -- whatever!" "You need to call your agent.") There has been some discussion of this show in the blogosphere (what female comics should/shouldn't do, what constitutes success); different strokes for different folks, and all that, but she's both serious and funny here, in a sort of late Lenny Bruce, concert-era mode. You play around with ideas long enough, some people will want to set you on fire. There is plenty of irony in Silverman's presentations, but her title is sincere: "I believe in miracles, though, I really do. They're obviously science-based, but they’re beyond my comprehension so to me they're miracles… and we are miracles.... Every single person in this room tonight... there was a time in history, a blip ago in the scope of history, when we were all microscopic specks -- that was far out, right? Everybody got deep." "You find humanity in the oddest of places," she says at the kicker to an opening riff on pornography. Stepping back and considering one disturbing shared thought, she notes, "I built a frame around it that forced you to not be able to blame me for saying it" and of another, "It went in my head and then I couldn't be alone with it." Comedy tonight.

"Nashville 2.0: The Rise of Americana" (PBS, Friday), "Austin City Limits: ACL Presents: Americana Music Festival 2013" (PBS, Sunday). A term that, in a musical context, gained traction in the mid-1990s as the name of a roots-friendly radio format -- whose constituent parts were already two decades old, in the primordial soup of country-rock and blues-rock -- "Americana" is having a(nother) moment. This weekend PBS airs two separate shows with overlapping content that give some sense of what can shelter under that umbrella, among the older generations and the younger. It's the familiar seesaw of the raw and the cooked, the slick versus the cowlicked, a back-to-analog yen for authenticity (even borrowed) that finds similar expression in hipster picklers, artisanal gin and the vinyl revival. The title "Nashville 2.0" is a bit of a misnomer; some of the bands here have nothing in particular to do with that city or its music (Mumford and Sons, who are English, open the film), and there's no substantive look at how Nashville fits the new music or the new music fits in with the old, or the more blatantly commercial new. And the way the word "America" sits in the mouth of the musicians interviewed here suggests that it's one they would not use to describe their own music but for the filmmakers' asking. And it all wanders a bit. But there's a lot of good singing and picking and sawing along the way. I haven't seen the "Austin City Limits" episode, but if other episodes and the coming attractions are any guide, it'll be all music. Several performers appear in both specials, including Emmylou Harris and Rodney Crowell, Buddy Miller and Jim Lauderdale, John Fullbright, Shovels & Rope, the Milk Carton Kids and Richard Thompson (another Englishman). "Nashville 2.0" also includes Carolina Chocolate Drops, the Mavericks,the Civil Wars, Jerry Douglas, Billy Bragg and the Lone Bellow; "Austin City Limits" adds Dr. John, Duane Eddy, Old Crow Medicine Show, Kelly Willis and Stephen Stills. (The latter show airs on KOCE at 1 a.m. Sunday morning, which is to say, very late Saturday night.)

robert.lloyd@latimes.com