If you love discount shopping, an overabundance of food, and the opportunity to wear sweat pants for four days straight as you watch football over Thanksgiving weekend, take a minute to give thanks for Sarah Josepha Hale. (The legal connection? She was married to a lawyer; yet another reason to praise Ms. Hale's brilliance!)
Ms. Hale was a fervent supporter of Thanksgiving, which wasn't actually a nationally recognized celebration until 1863. She wrote numerous letters to presidents (including Lincoln) petitioning them to make Thanksgiving Day a national holiday. Lincoln acquiesced. (According to abrahamlincolnonline.org, "Prior to this, each state scheduled its own Thanksgiving holiday at different times, mainly in New England and other Northern states.")
Where might you find an actual copy (like a pdf scan) of Lincoln's proclamation? Presidential proclamations are currently found in the Federal Register, but that government publication didn't exist until 1935 (July 26, 1935 to be exact). This is a case where the National Archives come to the rescue: Thanksgiving Proclamation.