(CNN) When Gov. Tom Wolf puts pen to paper later today, Pennsylvania will become the latest state to officially recognize Juneteenth, which commemorates the end of slavery in the United States.

Pennsylvania will join 45 other states and the District of Columbia in either marking the day as a state holiday or observance . That leaves just four states that don't recognize the holiday: Hawaii, North Dakota, South Dakota and Montana.

What is Juneteenth?

Juneteenth -- a blending of the words June and nineteenth -- is the oldest known US celebration of the end of slavery. It commemorates June 19, 1865. That's the day that Union Maj. Gen. Gordon Granger rode into Galveston, Texas, and told slaves of their emancipation from slavery.

Source: National Juneteenth Observance Foundation

"In accordance with a proclamation from the Executive of the United States, all slaves are free," Granger read to the crowd that day. It came more than two years after President Abraham Lincoln issued the Emancipation Proclamation.

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