What It Celebrates

Juneteenth Comes to Massachusetts
The holiday was not celebrated much outside of Texas until recently. With the move of Antiracism activism to the mainstream, it was slowly gaining a wider audience. In 2007, then Massachusetts governor Deval Patrick formerly recognized the holiday but it was not until 2020 that governor Charlie Baker made it an official Massachusetts state holiday. (Source)
Juneteenth Around the Country
The first state to make it a holiday was, naturally, Texas. Which it did in 1980. As of 2021, it is not a Federal holiday, only three states, North Dakota, South Dakota, and Hawaii do not recognize it as a state holiday. (Source)
Books
On Juneteenth by Annette Gordon-Reed
Juneteenth by Ralph Ellison
Further Reading and Listening
The Historical Legacy of Juneteenth – National Museum of African American History & Culture
The History and Meaning of Juneteenth – The Daily Podcast
The Birth of Juneteenth; Voices of the Enslaved – Library of Congress Blog
Celebrating Juneteenth: A Reading Of The Emancipation Proclamation – NPR