President Biden has signed a bill making Juneteenth a national holiday, commemorating the end of slavery in the Unite states. (CNN Instagram)

Juneteenth は、June と nineteenth の混成語です。

Juneteenth についてにニューヨークタイムズに説明がありました。

So You Want to Learn About Juneteenth?
On June 19, 1865, enslaved African-Americans in Galveston, Texas, were told they were free. A century and a half later, people in cities and towns across the U.S. continue to celebrate the occasion.

Juneteenth, an annual holiday commemorating the end of slavery in the United States, has been celebrated by African-Americans since the late 1800s. (The New York Times)

正式な名称は、Juneteenth National Independence Day となるそうです。