What comes between spring and fall? In the fashion world, it's not summer, it's "pre-fall." If all goes according to designers' and retailers' plans, it could be one of the most lucrative times of the year.

The term pre-fall refers to what used to be unglamorous, mostly commercial collections meant to tide shoppers over in between the big spring and fall seasons. They were typically filled with staples like sensible blazers, skirts and sweaters in unchallenging colors.

Pre-fall fashions were once low-key affairs, shown by private appointment—if the collections were shown at all.

But in the past few years, pre-fall has started stealing the spotlight with dramatic, red-carpet-ready looks and shows from high-profile designers including Oscar de la Renta and Karl Lagerfeld for Chanel that are on par with the designers' spring and fall collections.

For the past month, designer labels have been showing their new collections for pre-fall 2012. After a pause for the holidays, there will be more presentations from big names like Givenchy, including shows in which models just stand rather than walk a runway.

That's partly because despite its silly-seeming name, pre-fall is serious business. Pre-fall designs begin arriving in stores in May and June—two to three months before fall clothes start arriving—and generally stay on the floor until January.

This means the clothes are generally sold at full price for longer than those from the fall collections, before being marked down.

"Resort" collections, which come in between fall and spring and generally start entering stores in November, have also been rising in prominence and revenue, but pre-fall's timing has given it more celebrity buzz.

The pre-fall shows coincide with the early-year entertainment awards ceremonies including the Golden Globes and the Grammys.

"These shows have become the perfect marketing vehicle for pre-fall," says luxury goods consultant Robert Burke. "You show it in December and it's on the red carpet in January and February and in stores in May."

Pre-fall, in terms of a fashion season, is supposed to start being worn somewhere around mid- to late-August and early- to mid-September. The timing presents a weather-temperature challenge for shoppers and retailers.

"When these pieces hit the store in June or July, depending on the part of the country, it can be 80 or 90 degrees, so we're not only looking for really wow emotional pieces for the fall season with fur and leather, but also for lighter weights" and lighter colors, says Colleen Sherin, senior women's fashion director at Saks Fifth Avenue. It's something the retailer is especially concerned with as many of its stores are in the South, she says.

The in-between season also presents a challenge for designers. Michael Kors this month showed a coat worn with shorts, while Vera Wang showed dark-colored sleeveless fur coats and wool shrugs with light chiffon dresses in colorful spring-like prints. Mr. de la Renta showed short-sleeve outerwear and a silk merino "cardigan coat" with a mink collar.

The rise of mid-season collections began in earnest following the global recession of 2008, when even luxury shoppers stepped back from conspicuous spending. Retailers requested more deliveries from designer labels, hoping that rotating in more new merchandise would tempt shoppers to again open their pocketbooks.

Karl Lagerfeld's elaborate resort show in May 2010, held against the backdrop of Saint Tropez's harbor with models arriving by speedboat and celebrities like Diana Kruger flown in to sit front row, marked a watershed moment for these pre-season collections. Shortly after that, more designer labels put on shows or presentations and fashion magazines and websites eagerly covered them.

A spokeswoman for Oscar de la Renta, who showed 58 looks in his pre-fall runway show, said pre-fall is, in general, a big collection for the house "and very important sales-wise."

For Jason Wu, "the spring and fall collections are the main image drivers of the company," he says. But in terms of business, the pre-seasons are doing the heavy lifting.

For fall and winter, about 60% of orders from a group of key retailers is from the pre-fall collection, the designer says, while 40% is from the fall runway collection. Mr. Wu even took the opportunity during pre-fall to launch another bag line rather than wait until his fall collection.

With increasing up-to-the-second coverage, a trend shown on the runways for spring gets exhaustive exposure—and is worn by celebrities and tastemakers soon after the show is over. By the time that look reaches stores six or seven months later, it already looks and feels dated, says Catherine Moellering, executive vice president of Tobe, a New York-based trend-forecasting firm.

By January, "spring runway looks would have been worn already" by celebrities and personalities, says Mr. Wu.

That, of course, is a no-no for celebrities on the red carpet. For her appearance at the People's Choice Awards last January,Natalie Portman wore a button-front silk chiffon pleated dress from Mr. Wu's 2011 pre-fall collection.