What do people commemorate on Juneteenth?

Juneteenth (the designation for June nineteenth) marks the end of slavery in the United States. On that date in 1865, about two months after General Lee's surrender ended the War Between the States, federal (Union) troops landed at Galveston, Texas to enforce the Emancipation Proclamation issued by the late President Lincoln almost two and a half years prior. This plan was devised to overcome the expected anti-emancipation sentiment, but why it was delayed so long is still a matter of debate.