Consider this TV sweeps month Jerry-rigged. There's the "Seinfeld" finale on May 14, and then there's the other 27 days and nights of television.

The last "Seinfeld" has leap-frogged from being an hour of entertaining TV into a pop cultural landmark. NBC will milk it for all it's worth, charging more super-than-Super-Bowl ad rates ($2 million for a 30-second ad), and starting the "Sein-Off" at 8 p.m. that night with a series retrospective. The final episode itself will probably run longer than an hour, and NBC is tentatively planning to start the finale a little before 9 p.m. - but just when, NBC isn't sure yet.But wait - as the infomercials say - there's more. The May sweeps, which start tonight and run four weeks, have the usual mix of stunt casting, specials, four-hour movies and a level of overwrought shilling that would gag P.T. Barnum. The sweeps (in February, May and November) are used by local TV stations to set their ad rates, so the networks help them by trotting out big guns and new episodes.

One gimmick that stands out among the many is CBS's "compositing" old stars into current series. That's the process that has had John Wayne selling Coors beer and Fred Astaire selling vacuum cleaners by blending old and new footage. Now, Jack Benny will appear on "Cosby," Steve McQueen on "Walker, Texas Ranger," Lucille Ball and Desi Arnaz on "The Nanny," and young Dick Van Dyke as Rob Petrie will do a walk-on with old Dick Van Dyke on "Diagnosis: Murder."

For many viewers, just having fresh episodes of their favorite series, whether it's the randy teens of "Dawson's Creek" or the beleaguered docs of "ER," will be sufficient. There'll be virtually nothing but new episodes for the next four weeks, with dozens of big-wallop finales and cliffhangers locked and loaded.

Here's a look at some of what's ahead. Check local listings for correct times:

"Murphy Brown" (May 18 on Ch. 2) is the other long-running sitcom winding down, and while it's faded more than a bit, it can still crackle. The guest-star list includes Julia Roberts, George Clooney and Bette Midler, and no, Murphy doesn't die.

"Ally McBeal" (Monday on Ch. 13) and "The Practice" (Monday on Ch. 4) will stage a rare cross-network crossover, with characters from creator David E. Kelley's two legal-themed series visiting one another's worlds.

"Merlin" (Sunday-Monday on Ch. 5) tells the story of King Arthur from the viewpoint of his court wizard, with Sam Neill as the Wiz, Isabella Rossellini, Helena Bonham Carter, Martin Short (believe it or not, he steals the show) and a slew of special effects.

"Peter Benchley's Creature" (May 17-18 on Ch. 4) stars Craig T. Nelson. Not as the sea monster, but as the man who must stop it.

"Gargantua" (May 19 on Fox) is May's other monster mash, with Adam Baldwin as the man who must stop it.

"Mario Puzo's The Last Don II" (May 3 and May 5 on Ch. 2) should be subtitled "And this time we really mean it's the last." Danny Aiello, Kirstie Alley, Joe Mantegna and Jason Gedrick reprise their roles.

"The Long Island Incident" (May 3 on Ch. 5) stars Laurie Metcalf as Carolyn McCarthy, a nurse whose husband was killed by a gunman on a train and who ran successfully for Congress on a gun control platform.

"The X-Files" (May 17 on Ch. 13) always has big cliffhanger finales, but this one may be bigger than big. It sets up the summer "X-Files" feature film, which allegedly may answer some of the series' unanswered ques-tions.

"Witness to the Mob" (May 10-11 on Ch. 5) will explain why Nick Turturro was off "NYPD Blue" for about two months: He was starring as Sammy "The Bull" Gravano, who worked for John Gotti, then ratted him out.

"CBS: The First Fifty Years" (May 20 on Ch. 2) features an enormous who-was-who of CBS stars barrelling through five decades of clips and reminiscences, including Walter Cronkite, Alan Alda, the Smothers Brothers, Tom Snyder, Larry Hagman, Mike Wallace and Ron Howard.

"Miracle at Midnight" (May 17 on Ch. 4) stars Sam Waterston and Mia Farrow as a Danish couple who saved thousands of Jews during the Holocaust.

"Cher Remembers Sonny" (May 20 on Ch. 2), which pretty much says it all

"Lie Detector" (May 5 on Ch. 13) is a special in which host Marcia Clark will ask Mark Fuhrman questions while he's hooked to a poly-graph.

Enjoy it while it lasts. Come May 21, as if Merlin himself had waved a wand, the specials and original episodes will vanish, to be replaced by an endless bummer of reruns.