The first civil rights monument in the United States is having its diamond jubilee. The monument isn’t a temple, obelisk or sculpture. It’s a mural installed in the spring of 1942 at the entrance to the Interior Department’s basement cafeteria. The artwork, “An Incident in Contemporary American Life,” portrays the racially integrated audience at the Lincoln Memorial concert by the great African American singer Marian Anderson. In part, the mural reflects the racial attitudes of the era; it also conveys a radical message about civil rights that deserves a gala celebration to mark this, its 75th anniversary.

The Lincoln Memorial concert took place on April 9, 1939, Easter Sunday, two months after the Daughters of the American Revolution had refused to allow Anderson to sing in its venue, Constitution Hall. An audience of 75,000 gathered on the Mall, and millions more listened on the radio.

Soon afterward, national delegates of the DAR held a convention in Washington. When black activists considered picketing the event, establishment figures pressed them to reconsider. Eleanor Roosevelt wrote to NAACP leader Walter White, the driving force behind the Lincoln Memorial concert, urging him “to use your influence against this and leave well enough alone.” The activists agreed to back down.

Instead of picketing, they redirected their energy into the mural project. Leadership had shifted to federal arts administrator Edward Bruce, who was white. He had proposed the mural and became chair of the biracial mural committee, as The Post reported on April 18, 1939, only nine days after the concert.

Bruce had already decided on the location and selection process for the mural. “Secretary [Harold] Ickes has . . . expressed his willingness to have the painting installed in the Interior Department Building,” he told The Post. The paper added that a “national competition, open to all Americans, regardless of color, will be held to select the painter.” Bruce’s handpicked competition jury, which included just one black member, selected a white artist, Mitchell Jamieson of Kensington.

In his winning drawing, Jamieson’s focus on the audience at the Lincoln Memorial concert was apt and, in its way, subversive. Life-size figures are black and white, and, most provocatively for the period, a white woman and a black man stand side by side at the top of a staircase. Whether or not they are a couple, their proximity is a message of equality and integration — in a time when anti-miscegenation laws were still common.

In other respects, the message is conservative. The title does not mention Anderson, who appears as a speck in the distance. The tableau of the Mall is remote and timeless, and the piece does not reflect the racial controversy that preceded the concert. The winning drawing was idealistic, looking forward to a time when integrated audiences would be the norm.

Jamieson, however, had an artist’s temperament and skill. In his final, painted version, he added portraits that transformed the mural, making it a more radical statement on civil rights.

The African American man standing beside the white woman became Charles Hamilton Houston, head of legal affairs for the NAACP. A woman sitting on a chair below became educator Mary McLeod Bethune, the New Deal’s highest-ranking African American as director of Negro affairs for the National Youth Administration. Their inclusion was Jamieson’s way of restoring African American leadership on civil rights.

Sara A. Butler, an art historian at Roger Williams University, described the mural as “a shocking move for the period.” The Washington establishment, including Eleanor Roosevelt, had “endorsed a work of federal art that enshrined a key moment in the nation’s racial history and celebrated black protest,” she wrote.

Houston presented the mural at the official dedication on Jan. 6, 1943. The date marked specific progress in civil rights. According to an article in The Post, Houston “called attention” to the DAR’s decision to host Anderson for a charity concert that same evening, a reversal of the group’s previous ban on artists of color. A few hours after the dedication, Anderson sang to a sold-out, integrated audience of 4,000 at the DAR’s Constitution Hall.

The nation’s first civil rights monument is a gem. Its diamond jubilee calls for celebration not only by the Interior Department but also by the DAR next door and the National Museum of African American History and Culture down the street.