The siege of Richmond I believe. Or maybe the march on Fairfax. It happened in Virginia I know that.
Pretty sure I had heard a story where Thanksgiving was already a pretty strong tradition in the north. Or at least among certain Northerners. So much so it was condemned by Southerners for being an Abolitionist plot so they refused to acknowledge it at all.
Then the Civil War happened and Abe Lincoln consecrated it. Since I read it in a book I do not have I can’t cross reference anything but Wikipedia tho
Thanksgiving in the United States has been observed on differing dates. From the time of the Founding Fathers until the time of Lincoln, the date of observance varied from state to state. The final Thursday in November had become the customary date in most U.S. states by the beginning of the 19th century, coinciding with, and eventually superseding the holiday of Evacuation Day (commemorating the day the British exited the United States after the Revolutionary War).[29] Modern Thanksgiving was proclaimed for all states in 1863 by Abraham Lincoln. Influenced by Sarah Josepha Hale, who wrote letters to politicians for approximately 40 years advocating an official holiday, Lincoln set national Thanksgiving by proclamation for the final Thursday in November, explicitly in celebration of the bounties that had continued to fall on the Union and for the military successes in the war.[30] Because of the ongoing Civil War, a nationwide Thanksgiving celebration was not realized until Reconstruction was completed in the 1870s.
So we should really be celebrating less Pilgrims and Indians and more celebrating beating the shit out of the slave-based mode of production and pretend Aristocrats. So…
Hurrah for the choice of the Nation!