THE KENNEDY Center is observing its seventh anniversary this week in a way that may eventually have as large and effect as the Bernstein Mass with which the center opened seven years ago Sept. 8.

Beginning tomorrow and continuing - with minor interruptions - through next Sunday, the center will be housing not one but two major competitions. The first is its International Piano Competition for Excellence in the Performance of American Music. The second is a competition for American orchestral works that premiered in the past season.

These new contests will make Washington one of the dramatic centers in the world of international music competition. Added to the established Friday Morning Music Club Competitions and the J.S. Bach Competition, the Kennedy Center's contests, which focus on both the creative and the interpretive aspects in music, will provide Washingtonians with rare opportunities to hear some of the world's finest piano playing and the best of the new music of this country. That all of this will be heard in six free concerts adds a welcome dimension to the next few days.

Announced nearly a year ago, the International Piano Competition quickly drew this comment from Joseph Roddy, writing in the Rockefeller Foundation's newsletter: "The sponsors must find a set of judges from all over the world who are familiar enough with an unfamiliar body of music to gauge the quality of its performers."

In actually, the Kennedy Center had no difficulty in naming five judges who heard preliminary rounds in Los Angeles, Chicago, Washington, New York and Vienna, and who came up with not eight semifinalists, as originally projected, but, thanks to the caliber of those who entered the contest, with a field of 12.

Tomorrow starting at 5 p.m. in the Concert Hall, the first group of four competing are Christina Petrowska of Montreal; Jack Winerock of Lawrence, Kans.; Donna Coleman of Rochester, N.Y., and Robert Weirich of New Orleans. Prizes include over $30,000 in cash, plus such further inducements as concert tours and possible recording contracts.

On Wednesday at 5 p.m., a second group of four - Peter Lawson of Manchester, England; Aleck Karis of White Plains, N.Y.; Bradford Gowen of Bethesda, Md., and Adam Fellegi of Budapest, Hungary - will play. The last session of the semifinals will be Thursday at 5 p.m. Heard then will be Robert Taub of Metuchen, N.J.; Stephen Drury of Spokane, Wash.; Alan Feinberg of New York City, and Henry Martin of North Haven, Conn.

At each of these semifinal sessions, all of which are free to public, the competing pianist will play an hour-long program. The decisions of the judges will, in the language of the contest's initial announcement, "be based not only on excellent of performance, but on the effectiveness of the total program." During the preliminary rounds of the competition, the entire piano repertoire has been open to the contestants, along with the requirement that each entrant be prepared to perform major works from various periods in American piano writing.

At the close of Thursday's semifinals, the judges will announce the names of three pianists who will compete in the final rounds to be held in the Concert Hall on Saturday and Sunday. One finalist will play at 12:30 p.m. Saturday, a second at 3 p.m. The third and last will play at 2 p.m. next Sunday. The judges' decisions will be announced shortly thereafter, and it is likely that a major new career will be launched. One of the most intriguing elements in the contest is the word that the winning pianist may be awarded a further prize of $5,000 a second year, if he or she programs American music with sufficient success on the first tour.

One of the hospital aspects of this competition, which next year will be open to singers, is that the wide world of American music, ranging from the most unabashed romanticism to the most astringent avant-garde, will be made more familiar and more attractive to audiences who have heard less of it in recent years, at least on major concert series.

Hardly will the hurrahs for the winning pianists have ended in the Kennedy Center's Concert Hall next Sunday before another contest's final round will take place there. At 7:30 that evening, before a panel of three judges, the Peabody Conservatory Orchestra under the direction of Frederick Prausnitz, will play five orchestral works by American composers. Each of these had its world premiers within the past season. From the five, which were in turn selected by the judges from a final group of 10, the three that are considered the best will receive prize awards of $5,000, $2,000 and $500.

The five compositions to be played next Sunday are: Concerto for Orchestra, by Marc-Antonio Consoli of New York City; Concerto for Orchestra by Henri Lazarof of Los Angeles; Concerto for English Horn, by Vincent Persichetti of Philadelphia; Ricercari Notturni, by Stanislaw Skrowaczewski, the music director of the Minnesota Orchestra; and Adios, by Aurelio de la Vega of Northridge, Calif.

This second contest is named in honor of the late and renowned pianist Arthur Friedheim. It is funded by a grant from the Eric Friedheim Foundation, established by the artist's son.

Both of the Kennedy Center's new competitions have unusual and attractive factors. There is no age limit in the performers' contest, and the preliminary rounds saw pianists entering who were around 50 and 60 years of age.In the composition contest, anyone - composer, performer, listener - was free to nominate a work, as long as its premiere fell within the required period.

There are those who find it ironic, however, that as important a world premiere of major proportions as that of Leonard Bernstein's "Songfest," which was heard here last fall with the composer conducting the National Symphony Orchestra, was ruled out of contention by the judges. They rejected the score, "since it was decided that works including large choruses or consisting of song settings did not fall within the definition of 'orchestral music' as the judges interpret the intent of the competition."

That kind of post-factum ruling, which would eliminate Gustav Mahler's "Das Lied von der Erde," or Stephen Burton's recent orchestral triumph, "Ariel," sets up a highly arbitrary view of "orchestral music." It would have even kept Liszt's Faust Symphony from consideration. And what about the Ninth Symphony by Beethoven? You have to hope that Consoli, de la Vega, Lazarof, Persichetti or Skrowaczewski comes up with something so much better than "Songfest" that there can be no argument.

The thrust of the new competitions, however, is particularly appropriate to the Center's long-range objectives and is inested with great potential and excitement. It puts Washington in the same major league with New York, Brussels, and Moscow. The free concerts which make up the final rounds of both competitions should attract large numbers of listeners who enjoy keeping in touch with the newest and best in the world of music.