Timeline of Events Relating to the End of Slavery

 On July 8, slavery was effectively abolished in Massachusetts, with the ruling by the Massachusetts Supreme Court in the Commonwealth v. Jennison case. A slave named Quock Walker sued his owner for his freedom. The court ruled that he was free and the Commonwealth brought suit for wrongful imprisonment of Walker by Jennison. The court used the Massachusetts Declaration of Rights, that state "all men are born free and equal", as the basis for saying that slavery was abolished under the Massachusetts Constitution, which include the Declaration of Rights.

http://www.masshist.org/teaching-history/loc-slavery/essay.php?entry_id=504

In 1780, Pennsylvania became the first state to abolish slavery when it adopted a statute that provided for the freedom of every slave born after its enactment (once that individual reached the age of majority).  Massachusetts was the first to abolish slavery outright, doing so by judicial decree in 1783.

https://law.marquette.edu/facultyblog/2012/12/before-there-were-red-and-blue-states-there-were-free-states-and-slave-states/comment-page-1/