Celebrations in Old San Juan reflect Puerto Rico's age-old blending of cultures

* Check out the host of other seasonal events and shows offered free of charge to the public by the city of Old San Juan including a Three Kings Day concert. The San Juan Star, the island's leading English-language newspaper, offers a rundown of cultural happenings citywide in its Friday edition, complete with addresses, dates and times. Also consult Que Pasa, a free monthly booklet packed with information and published by the Puerto Rico Tourism Co. It features maps and photographs, current listings of cultural events and recreational activities. It's available in hotels and other tourist areas, or you can write for a copy of Que Pasa to Puerto Rico Tourism Co., 575 Fifth Ave., New York, N.Y. 10017; phone: [phone].

If you go . . .

San Juan roamings: Once on the brink of extinction, Old San Juan is today a thriving city with a wealth of beautifully preserved plazas, courtyards, churches and private homes that evoke its historic beginnings. Even the gleaming blue bricks of its narrow streets are cast of 18th century Spanish slag. Take a walking tour of the district starting at El Morro fortress, a masterpiece of Spanish military architecture, on the island's westernmost tip. Its sister fortification, San Cristobal, is within walking distance to the east. Both are maintained by the U.S. National Park Service and offer orientation and slide show programs.

In the heart of Old San Juan, visit the Gothic-influenced Church of San Jose, the second oldest in the Western Hemisphere. Next door is the Pablo Casals Museum, with its collection of the Spanish cello virtuoso's memorabilia. Casals, whose mother was born in Puerto Rico, settled here permanently in the mid-1950s, bringing with him the Casals Festival, an annual June event that attracts acclaimed musicians from around the world for two weeks of concertizing.

On the same street is the Casa Blanca, for 250 years the home of descendants of Ponce de Leon, the city's founder and seeker of the illusory fountain of youth. Now restored and operating as a museum, the complex is a priceless example of Spanish colonial architecture with its whitewashed facades and gardens.

Also scattered throughout the district are a number of small, intimate museums, each highlighting different aspect of Puerto Rican culture from earliest Indian civilizations and colonial architecture to folk and contemporary graphic arts. In addition to the Casals Museum and the Casa Blanca, visitors can stop by La Casa del Libro on Cristo Street, where a collection of rare volumes is on permanent display in an impeccably restored former mansion. For the holidays, the museum will feature a Christmas concert and exhibit. Original silk-screen Christmas cards by island artists are on sale at $2 to $5 throughout the season.

About Puerto Rico: As a Commonwealth of the United States, Puerto Rico shares our currency and Postal Service as well as many of our customs. American citizens need neither visa nor passport to gain entry here. And while Spanish is the official language, English study is obligatory from first grade and many people in San Juan speak it well.