Juneteenth is a holiday that is celebrated on June 19. 36 states in the United States observe it as an official state holiday. (This does not mean that it is a day off or anything like that, though. It just means the states officially have declared it to be a significant day.)
The purpose of this holiday is to commemorate the ending of slavery. The reason that this particular date is used is because that is the date on which slavery was ended in the state of Texas in 1865.
The holiday started out as just a Texas holiday, but it has come to be observed in many other states because, without it, there was no holiday to mark this very important event in US history.
In addition to the previous answer, it was the day a sailing ship landed in the port of Galveston. The captain of the ship informed the slaves on the dock that the war was over and they were no longer slaves but free men. This occured on June 19th, several months after the war was actually over. It took longer for Texas slaves to be informed primarily due to the slow communications, and I believe that the landowner didn't want to have to release their slaves. As I was informed by a black collegue, today it is celebrated by eating watermelon (since watermelon is full of water and was readily available in fields without having to spend time going to and from a well or other water souce...they - the slaves - could break open a watermelon and eat some then continue on working, losing less of their valuable working time), and drinking strawberry or red tinted fruit drinks (imitating the watermelon 'water'). Of course, there is the barbque, potato salad and chicken. Many families just enjoy the freedom of a day without work, groups going to a park to barbeque, spend time with friends, relaxing, etc. Remember, this holday began as a Texas holiday and has spread to 36 other states through the migration of Juneteenth celebrating Texans.