The Wikipedia article OP linked includes some details:

The Juneteenth flag was designed in 1997 by activist Ben Haith (also known as “Boston Ben”).[1] Haith displayed the first version of the Juneteenth flag in June 1997 at Boston’s John Eliot Square District. It was described by Patricia Smith of the Boston Globe as, “A banner adorned with sunbursts and flaming candles”.[2] A revised design was copyrighted by Haith in 2000.[3] Haith also founded the National Juneteenth Celebration Foundation (NJCF).[4] Lisa Jeanne Graf reports that as an illustrator she fine-tuned the design for the NJCF.[5]

The flag uses the colors red, white and blue of the American flag. Featured prominently in the center of the flag is a bursting star.[6] Running through the center of the flag horizontally, is an arc that is meant to symbolize the new horizon of opportunity for black people.[4] According to the president of the National Juneteenth Observance Foundation Steve Williams, the star is a “Bursting star of freedom.” Williams also states that the arch representing the horizon shows blue above and the red color below is symbolic of the ground soaked with blood; the blood which was shed of the Black Americans enslaved by the United States.[7] The red, white, and blue colors were meant to convey the message that all enslaved people in the United States – as well as their descendants – are American.[4] In 2007, the date “June 19, 1865” was added.[6]

Symbolism

The five-pointed star refers both to Texas (nicknamed the “Lone Star state”) and to the “freedom of African Americans in all 50 states”. Surrounding it is a 12-ray nova (or “new star”) representing a new beginning for all.[8]

Lincoln’s Emancipation Proclamation of January 1, 1863, declared that all slaves in the rebel Confederate States were free.[9] The State of Texas rejoined the Union with the end of the Civil War on April 9, 1865. On June 19, 1865, Union General Gordon Granger arrived at the port of Galveston, Texas and announced that slavery had been ended with General Order No. 3. The Juneteenth Flag has the date of June 19, 1865 displayed on it.

One year later freed slaves in Texas celebrated the first Juneteenth on June 19, and it was called “Jubilee Day”.[10]