Evacuation Day does not mark the elimination process that follows over-eating on Thanksgiving but celebrates November 25,1783 when the last British troops  left New York City at the end of the Revolutionary War. The holiday was once more popular in New York City than Thanksgiving. The celebration included parades, speeches and fireworks

New York City was the last piece of the 13 colonies surrendered by the British. The British occupation had traumatized the city.

From today's New York Daily News (linked above):

That occupation had not been kind to the city. Gen. Henry Clinton, the city’s commandant for four years, wanted “to gain the hearts and subdue the minds of America,” but his aide-de-camp, Lord Francis Rawdon, wrote of giving “free liberty to the soldiers to ravage at will” so that “these infatuated wretches” would understand “what a calamity war is.”

Rawdon’s view prevailed. British officers let their troops rape, murder and steal, while entertaining themselves with banquets, balls and “drunken orgies.” Food and firewood were hoarded for the troops; fires destroyed much of the city’s housing stock, leaving thousands in tent cities with no effective sanitation and setting off epidemics of cholera.