In what year did slavery become illegal in Pennsylvania?

You are essentially correct about the first part. In 1780, Pennsylvania passed a law directing that all slaves born in Pennsylvania after that year were to be freed once they reached the age of twenty eight. and that no further slaves were to be introduced into Pennsylvania after 1780.

The best calculations indicate that, thanks to this law, the last vestiges of slavery had been eliminated from Pennsylvania by 1808. There may have been a small handful still in slavery after that year but even most of those born before 1780 had been freed by that year. It is important to know that this 1780 law was passed as part of the general feeling that slavery was inconsistent with the principles of the Declaration of Independence. And, even those who held slaves were beginning to free them outright or to free them in their wills. A slave born in 1779 before this law was passed would have been 81 when the Civil War began and so could, theoretically, still be a slave. But, since the whole society in Pennsylvania had turned against slavery, it is doubtful that even one such person was still regarded as a slave by 1810 much less by 1860 or early 1861.

Your question points up the fact that slavery had existed in all thirteen of the colonies and then states that separated from Great Britain.. But, the revolutionary ideology worked against slavery and, in the northern states, where it had no strong economic roots slavery was either staged out of existence (as in Pennsylvania) or ended outright as happened in Massachusetts by a court decree that it was inconsistent with the state constitution that had been written in conformity with revolutionary ideology.

There were even efforts in the South to gradually eradicate slavery but the economic roots of slavery in the southern states proved strong enough to withstand the revolutionary ideology. And, slavery remained but became sectionalized and continued to exist only in southern or border states.

So, no one can say in exactly what year the last legally-held slave was alive in Pennsylvania but, for all practical purposes, slavery had been staged out of existence in Pennsylvania by 1808.

Offically, Sept. 22 1862. But only went into effect on Jan 1 1883.

Any person that went to school and paid attention would know what this date signifies.