Referring to June 19, 1865 which was two and a half years after the original issuance of the Emancipation Proclamation when Major General Gordon Granger arrived in Galveston, Texas and ordered the final enforcement of the Emancipation Proclamation because slave owners in Texas would not let the enslaved go free at the end of the American Civil War.
Juneteenth is the central event commemorated by the holiday which originally celebrated the end of slavery in Texas.

News of the Emancipation Proclamation had reached Texas and been covered in Texas newspapers, but due to the lack of Union military presence, it had not been enforced

General Order No. 3
The following is the text of the official recorded version of the order:

Head Quarters District of Texas

Galveston Texas June 19th 1865.

General Orders

No. 3.

The people of Texas are informed that, in accordance with a proclamation from the Executive of the United States, all slaves are free. This involves an absolute equality of personal rights and rights of property between former masters and slaves, and the connection heretofore existing between them becomes that between employer and hired labor.

The freedmen are advised to remain quietly at their present homes and work for wages. They are informed that they will not be allowed to collect at military posts and that they will not be supported in idleness either there or elsewhere.

By order of Major General Granger

F.W. Emery

Major A.A. Genl.
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Misconceptions
A common misconception holds that the Emancipation Proclamation freed all slaves in the United States, or that the General Order No. 3 on June 19, 1865, marked the end of slavery in the United States. In fact, the Thirteenth Amendment, ratified and proclaimed in December 1865, was the article that made slavery illegal in the United States nationwide, not the Emancipation Proclamation.

Another common misconception is that it took over two years for news of the Emancipation Proclamation to reach Texas, and that slaves did not know they had already been freed by it. In fact, news of the Proclamation had reached Texas long before 1865, and many slaves knew about Lincoln's order emancipating them, but they had not been freed since the Union army had yet to reach Texas to enforce the Proclamation. Only after the arrival of the Union army and General Order No. 3 was the Proclamation widely enforced in Texas.
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In the Civil War period, slavery came to an end in various areas of the United States at different times. Many enslaved Black people self-emancipated, demanded wages, stopped work, or took up arms against the Confederacy of slave states. In January 1865, Congress finally proposed the Thirteenth Amendment to the United States Constitution for the national abolition of slavery. By June 1865, almost all enslaved persons had been freed by the victorious Union Army or by state abolition laws. When the national abolition amendment was ratified in December, the remaining enslaved people in Delaware and in Kentucky were freed.

Early Juneteenth celebrations date back to 1866, at first involving church-centered community gatherings in Texas. They spread across the South among newly freed African Americans and their descendants and became more commercialized in the 1920s and 1930s, often centering on a food festival. Participants in the Great Migration brought these celebrations to the rest of the country.

Early celebrations consisted of baseball, fishing, and rodeos. African Americans were often prohibited from using public facilities for their celebrations, so they were often held at churches or near water. Celebrations were characterized by elaborate large meals and people wearing their best clothing. It was common for formerly enslaved people and their descendants to make a pilgrimage to Galveston.

As early festivals received news coverage, Janice Hume and Noah Arceneaux consider that they "served to assimilate African American memories within the dominant 'American story'".

Modern observance is primarily in local celebrations. In many places, Juneteenth has become a multicultural holiday. Traditions include public readings of the Emancipation Proclamation which promised freedom, singing traditional songs such as "Swing Low, Sweet Chariot" and "Lift Every Voice and Sing", and reading of works by noted African-American writers, such as Ralph Ellison and Maya Angelou Celebrations include picnics, rodeos, street fairs, cookouts, family reunions, park parties, historical reenactments, blues festivals, and Miss Juneteenth contests.

Red food and drinks are traditional during the celebrations, including red velvet cake and strawberry soda, with red meant to represent resilience and joy.

Juneteenth celebrations often include lectures and exhibitions on African American culture. The modern holiday places much emphasis on teaching about African American heritage. Karen M. Thomas wrote in Emerge that "community leaders have latched on to [Juneteenth] to help instill a sense of heritage and pride in black youth." Celebrations are commonly accompanied by voter registration efforts, the performing of plays, and retelling stories.[24] The holiday is also a celebration of soul food and other food with African American influences. Major news networks host specials and marathons on national outlets featuring prominent Black voices

During the Civil Rights Movement of the 1960s, these celebrations were eclipsed by the nonviolent determination to achieve civil rights, but grew in popularity again in the 1970s with a focus on African American freedom and African American arts. Beginning with Texas by proclamation in 1938, and by legislation in 1979, every U.S. state and the District of Columbia has formally recognized the holiday in some way.

Juneteenth is also celebrated by the Mascogos, descendants of Black Seminoles who escaped from slavery in 1852 and settled in Coahuila, Mexico. The Black Seminoles of Nacimiento in Mexico hold a festival and reunion known as el Día de los Negros on June 19.

Many former British colonies celebrate Emancipation Day on August 1, commemorating the Slavery Abolition Act 1833. Since 2021, the United Nations has designated August 31 as the International Day for People of African Descent.

The day was recognized as a federal holiday in 2021, when the 117th U.S. Congress enacted and President Joe Biden signed the Juneteenth National Independence Day Act into law. Juneteenth became the first new federal holiday since Martin Luther King Jr. Day was adopted in 1983.