Walnut Street on Wednesday nights: Crowds of twentysomethings cluster. Women in breezy sundresses flirt. Midweek merrymakers stand pressed against the front windows of tiny bars. The din is constant.

Why is there so much action in Center City on hump day?

Because Wednesday is the new Friday.

(Unfortunately, this doesn't mean Thursday is the new Saturday. We wish.)

But thanks to Center City Sips - an organized network of local happy hours - Wednesday evenings have evolved into the night for young professionals to nosh and network before they head off to the Shore, spend time with their families, or finish up do-it-yourself projects during the weekend.

"I plan my Wednesdays so I don't have to work in the afternoons," said Meredith Syles, 25. Syles and 11 of her friends filled two tables in front of Brasserie Perrier one recent evening.

"Wednesday night is my night out. Thursday night I'm packing, and Friday I'm at the Shore. . . . We get here early and just drink water until the drink specials start."

Brasserie is definitely one of the Wednesday-night hot spots, but other places like 1225 Raw Sushi & Saki Lounge and El Vez restaurant in Center City draw sizable crowds, lured by discounted drinks and the chance to meet the right people.

The merriment, however, doesn't stop when the price of drinks goes up at 7. In the last three years, promoters, taking advantage of the bodies milling around Walnut Street, started hosting dance parties as a follow-up to Sips.

The Walnut Room and Vango Lounge & Skybar, a barely year-old Asian fusion restaurant, host parties that feature house music as well as old-school R&B beats.

Luxe Lounge, a new upscale venue, held its inaugural VIP event on a recent Wednesday night. City officials. Teachers. Bankers. The place was packed.

And local party promoter Tommy Up will launch his weekly Wednesday night bash for the hipster crowd tonight at Pearl, another Center City restaurant and nightclub.

"Wednesday is the night for people who feel the nightspots don't belong to them on Fridays and Saturdays," said Up, who views weekend evenings as more for people who work outside the city - he jokingly referred to them as the Schuylkill Expressway and Ben Franklin Bridge crowd.

"When I go out on Wednesday nights, I see people that I know . . . people that I'm friends with," he said. "It's a much more intimate evening."

For Millennials and GenXers whose formidable party years were filled with weekends that began on Thursdays, midweek soirees aren't a new phenomenon. Old party habits are hard to break, and by midweek, these now-older professionals and parents are still jittery to get out. That's especially true when the weather is nice.

Married GenXers, and those with kids, say it's easier to arrange child care midweek. Others just bring their babies along.

Three-and-a-half-week-old Olive Grace Cray was hanging out with parents Joe and Joanne from South Philadelphia one recent Wednesday in front of Brasserie Perrier.

"It's a nice way to get out and reconnect with people," Joanne Cray said.

The shift to midweek functions can be attributed to our changing work lives that no longer revolve around the basic Monday-to-Friday, 9-to-5 schedule, explained Frank Farley, a Temple University psychologist who studies culture, social life and personality.

That, plus technology in the form of cell phones, laptops and PDAs, helps people make business deals and keep their bosses up to date while enjoying a cocktail earlier in the day.

"The midweek social surge makes complete sense," Farley said. "We are on all the time, but at the same time, the social barriers are crumbling."

The first Center City Sips were held Tuesday evenings in 2005. But that first year was a flop, according to Michelle Shannon, vice president of marketing for the Center City District.

The next year, the event was moved to Wednesdays, with new prices: mixed drinks $4, wine $3, beer $2. Now, Shannon said, the 65 restaurants that participate in Center City Sips see their profits double on Wednesday nights in the summer.

"Historically, we've promoted Wednesday night as a late shopping night in Philadelphia," she said. "But now it's the most exciting night of the week."

Want more evidence that midweek is when the cool kids come out to play?

Some after-work socializing starts as early as Tuesdays, said Brett Silver, founder of SocialPhilly.com, an online society page and calendar of events that he renamed EventUpdates.com. In fact, Silver said, Tuesdays and Wednesdays are the most popular nights for social networking, while fund-raisers and red-carpet openings tend to be held Thursdays.

"In cities like New York, Miami and Chicago, the weekends are for amateurs," said Jared Soble, 28, who was out recently on Wednesday and Thursday in Center City. Soble travels up and down the East coast as a sales executive for Fiji Water.

This summer, the Old City District launched its version of Sips, naming it Old City Thursdays. About 25 restaurants are participating in the happy-hour specials, and local boutiques will be open until 9.

Within the last two years, even Sunday afternoons are bubbling into an early-evening bash.

Local DJ Lee Jones hosts a weekly block party at the Bubble House, which draws several hundred people to the alley behind 36th and Sansom. Tommy Up runs a similar event at Vango. Both events draw GenXers and their young families.

So open up that calendar. Friday and Saturday no longer corner the market for fun. Yet with all these new days to squeeze in key socializing, it's unfortunate that Monday is still the same old Monday.

Where's the party?

To find a complete list of restaurants that participate in Wednesday night's Center City Sips, go to www.centercityphila.org/life, or call the Center City District at 215-440-5500.

For more information on Old City Thursdays, go to www.oldcitydistrict.org, or call 215-592-7929.