From their event page:

Juneteenth marks our country’s second Independence Day. Juneteenth is the oldest nationally celebrated commemoration of the ending slavery in the United States. On June 19, 1865, over two years after the Emancipation Proclamation was signed and nearly five months after the 13th Amendment was proposed (it would not be ratified until December 6, 1865), enslaved people in Texas learned that they were free.

Admission to the Center is free on Juneteenth to celebrate this national holiday and rejoice in the celebration of those who fought to secure freedom for all. Museum programs will highlight the history of the holiday and stories of freedom fighters from the Reconstruction era like Frederick Douglass, Harriet Tubman, Frances Ellen Watkins Harper, and more.