Juneteenth commemorates the end of slavery in the United States. You can read about the origins of the holiday on Wikipedia or on Juneteenth.com.
Another page on the Juneteenth.com website has a number of helpful suggestions on how this important day may properly be observed.
But none involve martinis. However, since today is also National Martini Day, and if you don't feel like following any of those suggestions... why not toast Juneteenth with a martini? Or two.