Who will turn the world on with her smile, now that "The Mary Tyler Moore Show" is coming to an end? Who will take a nothing day, and suddenly make it all seem worthwhile? Laverne and Shirley? No, somehow it's not the same.

Inevitabilities have to be faced sooner or later, and for this one, it's sooner. It's tonight. As it must to all situation comedies, death comes to "The Mary Tyler Moore Show." The last episode, No. 168, will be televised at 8 o'clock on Channel 9 and other CBS stations.

The show will make you want to get right down there on the floor with Ted Baxter and blubber like a baby. It may have been hard to hold back the tears when John-Boy (Richard Thomas) said goodbye to Walton's Mountain, also on CBS, earlier this week, but that was just a warm-up for tonight's ordeal.

In the final chapter, WJM-TV is sold to a new owner who fires the entire newsroom staff, with one screamingly ironic exception. It's a funny, heartbreaking, heartfelt conclusion to a television series that has expanded the dimensions of the form and remained, for the most part, a model of civility in a medium that often seems populated largely by a medium that often seems populated largely by louts.

When Edward Asner as Lou Grant says tonight to his fellow characters, "I treasure you people," he is speaking for all of us. Watching the farewells at the end of the show makes one feel like Dorothy bidding her bye-byes to the people of Oz, but without an Auntie Em to go back to.

The series started inauspiciously in September 1970. One New York TV critic dismissed the premiere in a few grumpy paragraphs, called it a "ridiculous item." In its preview of the new fall TV season, Newsweek didn't even mention the show. It was nearly canceled in its first year.

Funny how these things happen.

Now the program has developed an uncommon loyalty, even among people who otherwise claim to hate television. Esquire magazine recently published a cover story full of sobs from famous anguished "Moore" fans, and this week's New Yorker includes a cartoon showing the immortal angry viewer about to chuck his TV set over a cliff following the telecast of the show's last episode.

In truth, the program's writers did some training this season, nudging characters into arch caricature at times. But the finale is a brilliant juggle of tense hilarity and cathartic sentiment. Cloris Leachman as Phyllis and Valerie Harper as Rhoda return from Spinoff Land for the last rites, joining Moore, Asner, Ted Knight, Gavin MacLeod, Betty White and Georgia Engel - "the best cast, ever," according to Moore, in a well-earned curtain call after the final commercial.

The last episode of "The Mary Tyler Moore Show" is probably the funniest sad goodbye that 30 million people ever said to friends good and true and vicarious.