It had previously been celebrated only in New England. Each state scheduled its own holiday, some as early as October and others as late as January; it was largely unknown in the American South. Her advocacy for the national holiday began in 1846 and lasted 17 years before it was successful. In support of the proposed national holiday, she wrote letters to five Presidents of the United States.
Her initial letters failed to persuade, but the letter she wrote to Lincoln did convince him to support legislation establishing a national holiday of Thanksgiving in 1863. From Wikipedia
Have a Happy Thanksgiving!