Heather Cox Richardson writes:
In October 1863, President Lincoln declared the second national day of Thanksgiving. In the past year, he declared, the nation had been blessed.
In the midst of a civil war of unequaled magnitude and severity, he wrote, Americans had maintained their laws and their institutions, and kept foreign countries from meddling with their nation. They had paid for the war as they went, refusing to permit the destruction to cripple the economy. Instead, as they funded the war, they had also advanced farming, industry, mining, and shipping. Immigrants had poured in to replace the men lost on the battlefield, and the economy was booming. And Lincoln had recently promised that the government would end slavery once and for all. The country, he predicted, “with a large increase of freedom,” would survive, stronger and more prosperous, than ever. The President invited Americans “in every part of the United States, and also those who are at sea, and those who are sojourning in foreign lands” to observe the last Thursday of November as a day of Thanksgiving.
This is a good reason.