When Harry W. Henderson and his wife arrived at the Kennedy Center to attend a symphony concert last Friday afternoon, they found the center's garage full.

Harry had little choice except to park at the Watergate and pay $5 for what would cost him $2.50 at the Kennedy Center.

"Later," Harry reports, "I was told that the KC on weekdays rents out space on a monthly basis to office workers and others in Foggy Bottom, thereby reducing parking for KC patrons. This is heresay but it sounded believable. We have been pushed around before by the Washington bureaucracy."

It's true, Harry.

Thomas R. Kendrick, director of operations at the center, explains that the garage has three levels, with 350 spaces on Level A, 450 on Level B and 600 on Level C. Spaces on Levels A and B are never rented by the month, but for $52.70 a month one can park on Level C in the daytime, Monday through Friday. The monthly rate does not permit night or weekend parking.

The reason monthly parking is sold to commuters is that Congress wants the center to "utilize all possible sources of revenue."

Nights and weekends, the 1,400 parking spaces in the garage can fill up quickly, especially when 7,000 people attend performances in the five theaters on the same night.

However, the building's attractions generate little demand for parking space in the morning. In season, there's some tourist trade, but that's about all. Demand for parking spaces is greater after noon because there is frequently a matinee performance in one theater or another.

A matinee in one of the smaller theaters can be handled easily by the 800 spaces on Levels A and B. However, the symphony matinee does cause problems because the concert hall is the largest of the five theaters. It seats 2,800.

The General Accounting Office has never assumed that the center's garage was meant to guarantee a parking space for every theatergoer. GAO has repeatedly urged the center to find a way to raise more revenue from its garage, and especially from spaces not needed for theatergoers in the daytime. The Kennedy Center management has responded by selling an average of 440 spaces, or $23,188 worth, to commuters each month -- a hardheaded, businesslike decision that I suspect Mr. Reagan would applaud.

Kendrick says, "In spite of the need for revenue, we try to keep our parking fees low. We realize that we're housed in a national monument, and we want tourists to be able to visit the building without feeling that their government overcharged them."

Now that the situation has been explained, I think the message for Harry Henderson and for all of us is: If we hope to save steps and save money by parking in the Kennedy Center garage, we must get there early