The National Symphony Orchestra is celebrating Rudolf Serkin's 80th birthday a bit prematurely tonight. The actual date will be March 28--one day after Mstislav Rostropovich's 56th. But a birthday as significant as this one should take quite a while to celebrate, with more than one city--perhaps more than one continent--getting into the act.

The public is invited to the celebration (which also happens to be a concert for the benefit of the NSO Pension Fund), but in a way it will be a family celebration. When Serkin's piano engages in extended dialogues with the NSO woodwinds--a frequent event in Mozart piano concerti--one of the chief spokesmen for the woodwind point of view on the music will be his son-in-law, NSO principal oboist Rudolph Vrbsky, who worked with Serkin for years at the Marlboro summer music festival and married his daughter, Judith. To get as much mileage as possible out of the Serkin clan on this occasion, the orchestra also will be performing Rossini's Overture to "La Scala di Seta" ("The Silken Lader")--chosen specifically because of its prominent oboe part. Also on the program will be Shostakovich's Fifth Symphony--presumably because it is one of the most spectacular items in the repertoire of Rostropovich and the NSO.

Serkin's connection with the National Symphony extends over half his lifetime--long before he became an orchestral in-law. He has been performing in Washington almost as long as the NSO, having made his debut here in 1933 when the orchestra was 2 years old. On that occasion, he performed in the Coolidge Auditorium of the Library of Congress with the Busch String Quartet (whose leader, Adolph Busch, was his father-in-law.) He did not get around to playing with the NSO until 1942, when they did Brahms' First Piano Concerto. Since then, he has been a frequent and welcome guest--notably during the opening week of Rostropovich's tenure as music director in the 1977-78 season. In March of that season, Serkin and the orchestra celebrated his 75th birthday with a concert in Carnegie Hall. The last time he performed at an NSO Pension Fund concert was in 1981--Beethoven's "Emperor" Concerto. Nobody complains about hearing Serkin in that repertoire, but tonight's choice is, in a way, more appropriate for the occasion. Mozart composed his 19th Piano Concerto for a fund-raising concert, with himself as the beneficiary, at a time when his financial condition was no better than the NSO's is today.

Serkin has made two specially notable appearances in Washington during the past year: last month in a dazzling performance of the last three Beethoven piano sonatas and in July in a cello-and-piano recital with Rostropovich for the benefit of Wolf Trap. The two Brahms sonatas, which shared that program with Beethoven's Sonata in A, Op. 69, will be released by Deutsche Grammophon in the near future.

Serkin's 80th birthday comes exactly a week after the 298th of Johann Sebastian Bach--appropriately enough, since the spirit of Bach is a key to Serkin's art. When he began performing in Europe (in 1915, at the age of 12), his most notable specialty was Bach. Since those days, musicology and the career of harpsichordist Wanda Landowska have cast a theoretical shadow over performances of Bach on the piano; his basic keyboard instruments were organ and harpsichord. But no criticism of what pianists do to Bach can obscure the fact that Bach does very good things to some pianists.

In Serkin's case, the music and the pianist's temperament meshed to produce a style of superb clarity, a masterful grasp of musical structures and sequential logic, a basic objectivity of style that enhances the music's power by letting it speak for itself without added and irrelevant rhetoric. If these virtues are particularly appropriate for Bach, Serkin's career has demonstrated that they work equally well in the music of Mozart, Beethoven, Schubert and Brahms.

Serkin's actual birthday, March 28, will be the date of the second NSO Pension Fund concert this year. The pianist--a very shy, retiring person among strangers when he is not seated at a keyboard--has not made it clear what he will be doing on that evening, but he will not be playing in Washington with Rostropovich--or in Boston with Ozawa or in London with Abbado, who both tried to get him for this occasion. Instead, the NSO Pension Fund program for March 28 is highlighted by Saint-Sae ns' "Carnival of the Animals," with Nancy Reagan (substituting for the late Princess Grace of Monaco) reading the Ogden Nash verses.