A gala 300th birthday bash for City of Brotherly Love

In 1682 a visionary Quaker named William Penn set aside 1,280 acres on a peninsula between the Delaware and the Schuylkill Rivers for his ''City of Brotherly Love.'' Now, in a series of gala events called Century IV, Philadelphians are preparing to celebrate the cultural richness and diversity that have been steadily growing in their city ever since.

Festivities will start April 25 when the Queen Elizabeth 2, the world's only transatlantic superliner, enters the Port of Philadelphia. The voyage of the QE2 from Southampton, England, to the city is meant to commemorate Penn's founding voyage from England to the New World in 1682.

A number of events are scheduled both on and off the 67,000-ton Cunard Line flagship while it lingers in port for three days. Fronting the docking area at Packer Avenue terminal is a 500,000-square-foot amphitheater specially constructed for the occasion. It can accommodate a quarter of a million people. Each afternoon the ship is in port there will be Century IV ceremonies and a variety of entertainment performed on an adjacent covered stage.

When the QE2 makes her return voyage back to Southampton April 28, Philadelphia will gear up for a month-long celebration of local restaurants and cuisine.

The first culinary event, held May 1 and 2, will honor the food that Philadelphians ate during Colonial times. Special tours of the eight historic mansions in Fairmount Park will enable participants to sample dozens of Colonial recipes while visiting the beautifully restored 18th-century homes.

The next event, the Philadelphia Folk Fair held in the Philadelphia Civic Center May 7-9, honors the diversity of ethnic food that immigrants have brought to the city. The Folk Fair will offer 50 types of cuisine and will include dancers, folk singers, and craftsmen as well.

On May 23, 100 of the city's restaurants will participate in the Philadelphia Restaurant Festival. Benjamin Franklin Parkway will be the scene of an extra-long sidewalk cafe in which pedestrians can stroll along and sample a smorgasbord of local restaurant fare.

Then on June 17 some 40 tall ships sail up the Delaware River to Penn's Landing. Heralded by a chorus of bells, horns, and whistles and accompanied by a flotilla of small craft, masted vessels from more than a dozen countries will form a ''Parade of Sail'' lasting from about 8 a.m. to 3 p.m.

During the next three days that the ships are in port, they will be open to the public. On the morning of June 18 a parade of ships' crews in national uniforms will proceed from Penn's Landing to Independence Hall. An awards ceremony at Independence Hall, with honorary commodore Walter Cronkite officiating, will then honor the winners of a sailing race from Venezuela to a finish line south of Philadelphia scheduled for late May. On the evening of June 20 the waterfront area will be the scene of a fireworks extravaganza.

From June 30 to July 11 Philadelphia will host the American Music Festival, which the city hopes to make an annual event. On stages set up throughout the city, frequent concerts will reflect all types of American music -- jazz, rock, opera, ballet, country and Western, bluegrass, rhythm and blues, gospel, folk, and pop. Native Philadelphians such as Bill Cosby, David Brenner, and Frankie Avalon are scheduled to perform in their old neighborhoods. Among the festivities will also be a 1950s-style sock hop in honor of that Philadelphia-originated TV show, ''American Bandstand.''

During three weekends in August, some 30 neighborhood civic organizations will host block parties all over the city. Shuttle buses will be available to transport visitors to the celebrations, featuring ethnic foods, parades, historic tours and exhibits, and dance and musical performances.

Finally, from Oct. 23 to 31, a week will be set aside to commemorate William Penn and his plan for Philadelphia as a city of religious freedom. During the week a re-creation of the barge Penn sailed up the Delaware River will make a similar journey, stopping at Chester, Pa., and Pennsbury Manor, Penn's country estate. On Oct. 24 an interfaith service will be held on Penn's Landing. Later in the day will come an extravaganza of fireworks and ice cream and cake.

In addition to these major events, dozens of special museum exhibits, tours, film festivals, and other happenings will be taking place in the city all during the months celebrating Century IV. For more detailed information on Century IV contact the Philadelphia Convention and Visitors Bureau, 1525 John F. Kennedy Boulevard, Philadelphia, Pa. 19102.