Tomorrow morning, a giant that has been sleeping on the Washington skyline will awaken.

With a parade of horses, clowns and marching bands cavorting at its feet, the renovated old Post Office on Pennsylvania Avenue NW will open its doors after decades of neglect, unveiling a stylized arcade of 50 shops and restaurants overlooking a performing arts stage.

The three-story mall, called the Pavilion, has been touted by both city officials and the local retail establishment as a means of breathing new and much-needed life into the Federal Triangle, traditionally one of the sleepiest business areas of the city. And as an added attraction, when the U.S. Park Service opens the structure's clock tower to the public next spring, it will become Washington's second-highest observation point after the Washington monument.

With an elaborate ribbon-cutting ceremony at noon tomorrow, the Pavilion's developer hopes to "unwrap a marketplace of excitement to dazzle downtown" and revitalize an area that has been neglected by retail stores and restaurants.

"There's a void here I think we can fill," says Charles C. G. Evans, president of the Evans Development Co. and developer and manager of the project. "I think our opening is going to be a tremendous catalyst to renew downtown business."

the Pavilion's shops, as well as its food kiosks, will be small--about 600 square feet each. Therefore, it will lend itself to customer browsing and impulse-buying, Evans notes, rather than to large-scale shopping sprees for necessity items. "This is not for the serious kind of wardrobe shopping. You don't come here to buy a $400 suit. You come here to buy gifts, impulse items and accessories--costume jewelry and sweaters that you can't find in any other place."

Garry Curtis, the Greater Washington Board of Trade retail bureau manager, agrees that the Pavilion "won't have a significant impact on the retail sales volume" of the city but he notes that the heavy performing-arts schedule and observation tower should be a good draw for residents and tourists. "From a retailing point of view, anything that brings people downtown sure is a plus."

Henry A. Berliner Jr., chairman of the Pennsylvania Avenue Development Corp., agrees. "We're not used to seeing stores and restaurants on the avenue. That is the critical point of that building. It enhances the whole idea that the avenue is a seven-day-a-week, 24-hour-a-day place for people--not just for government employes or tourists--to eat and shop. It will bring people into the city."

The Federal Triangle should have an even stronger attraction when National Theater--just two blocks away--reopens in mid-January, Berliner predicts. Business also is expected to be spurred by the opening of the Rouse Co.'s National Place Project adjacent to the theater. With 2 1/2 times the Pavilion's retail space, National Place will have a little more than 100 stores and restaurants. Rouse is planning to open half the project in late February, with the rest expected to be completed by the fall of 1984.

The viability of "any one of these projects by itself might be questionable," says John McKoy, director of planning for the District of Columbia. For instance, McKoy notes, the two restaurants already operating in the area--the Bread Oven and J. J. Mellon--"are not having very good pickings in the evening. If they are making it, they are doing it only on noontime activity."

Yet, McKoy says, together, these projects "should feed off one another so that over six to nine months, they should be able to sustain themselves."

"The more there is, the better," says Evans, when asked if the National Place would hurt the Pavilion. onetheless, Evans quickly adds, "no one can match the special nature of this building and location," only 100 feet away from a Metro station and a half block away from the Smithsonian.

The granite Romanesque building--with its detailed cornices and fancy filigree--has been considered an eyesore almost since the day it opened in 1899. The Post Office quickly became unsatisfied with it, saying the vaulted interior courtyards weren't appropriate to sorting mail.

Then, in the early '30s, the government concluded that the building detracted from the rest of the classical revival architecture found in the Federal Triangle and downtown, and decided to tear down the Old Post Office building. But the government ran out of money, and the building subsequently became a rundown catch-all for government agencies that had nowhere else to go. The preservationist group Don't Tear It Down later took up the battle to save the building.

Thanks to the passage of the Public Buildings Cooperative Use Act of 1976, the Old Post Office is no longer an outcast. The law permitted the government for the first time to allow private enterprise ventures in public buildings. The opening of the Pavilion will mark the first such project for the government. In addition to housing the Pavilion, the 10-story building is also home to offices of the National Endowment for the Arts and Humanities and the Institute of Museum Services.

For use of the government building, Evans will pay the General Services Administration $197,520 a year--or almost $3.30 per square foot--plus one-tenth of the sales percentage he receives from his tenants. As rents increase, Evans will have to pay GSA 10 percent of any increase.

While the rental fee may seem small, Evans argues that it is fair because "none of the work done for the Pavilion was funded by anyone other than us. The $10 million we invested was all ours."

Evans says he and his partners--who also have renovated the Arcade in Providence, R.I., and Baystate West in Springfield, Mass., took great pains in selecting their tenants.

"We spent as much time in selecting our 45 to 50 tenants as most developers would have in selecting 125 stores for a large regional shopping center. We were very selective, seeking owner-operated stores run by entrepreneurs" who are personally involved in their businesses. Restaurant chains and department-store chains were turned down because "that isn't what we're all about," Evans said.

For the four restaurants, Evans and his partners reviewed menus to make sure none conflicted. They say they were equally picky about the 15 carry-out kiosks, in one case holding several tastings of food from the menu for a Mideastern kiosk.

They also tried to be selective in their choice of stores. For instance, after concluding that they couldn't find a costume-jewelry store "with the standards we want," Evans' wife convinced him and his partners to open a jewelry store owned by their company. They hired a saleswoman who had been with Gucci here, began ordering jewelry and within three months created a new store called Impulse, which should be ready to open tomorrow.

On the other hand, when the developers found tenants they really wanted, they eagerly wooed them. As Jak Badenhoop, owner of the It's Only Natural clothing store, explains: "It didn't look like it would be easy for me, a pretty small operator, to open," especially considering that he was based in Florida.

"It is not easy for me to go into projects out of my territory," Badenhoop said. "But they helped me find contractors here and gave me a list of architects who would be good for my kind of store."

Additionally, Badenhoop said, Evans was willing to make exceptions in the terms of payments, making it possible for Badenhoop to delay payments that normally would be due immediately after opening--at a time when he needs cash to build up winter inventory in all of his five stores--until closer to Christmas when the cash flow is better.

Evans says 98 percent of the project is leased, leaving space for one restaurant and three retail stores that he hopes to fill shortly after opening.

Evans, who left the Rouse Co. to be on his own, says the statistics alone indicate that the Pavilion will be a business success: Within four blocks of the project, there are 115,000 office workers who have few places to eat or shop--during lunch and after work.

Additionally, Evans says, the area gets about 11 million tourists a year "who only have hot-dog carts and mobile T-shirt shops" on which to fill their stomachs and satisfy their impulse-buying urges. If the Pavilion captures only 1 percent of the visitor dollars available for food, beverage and retail purchases, the project will be successful, Evans says.

Those projections are a prime attraction to many of the Pavilion's tenants, including Len Goodman, head of Arius Tiles, a New Mexico company that sells handcrafted ceramic tiles.

Established at the renovated Faneuil Hall in Boston, Goodman says he has been sought out by a number of other developers. But, he says, he felt other popular projects, including Baltimore's Harborplace and New York's South Street Seaport Marketplace was "not upscale enough" or "didn't provide enough traffic."

Eager to establish another base in the East, Goodman decided to see what Washington had to offer. Driving around town, he "stumbled into the Pavilion and made a deal" after being almost instantly impressed by the high tourist and employe base, among other things.

Some of the Pavilion's tenants are excited for more than business reasons. Robert Green, owner of the Dupont Circle restaurant called Bootsie, Winkey & Miss Maud, will open a new eating place called Fitch, Fox & Brown in the Pavilion. Long before its opening, Green was pushing the city to close Pennsylvania Avenue on New Year's Eve "so we can celebrate the new year at the clock tower." Green maintains that "so many things can happen in this city with the tower as its centerpoint."

Clearly, that is what Evans is hoping for. In the meantime, his company is pursuing two other projects in the area. Although he won't give details, saying they are "in the pre-development stage," he says they involve "specialty retail opportunities here."

"This city" he says, "is under-stored."