From Time Magazine's "Top 10 Things You Didn't Know About Thanksgiving"
FDR learned the hard way not to mess with some traditions. In 1939, the President declared that Americans should celebrate the annual feast one week early, hoping the decision would spur retail sales during the Great Depression. But Americans did not react kindly to the New Deal meal. Some took to the streets while others took to name-calling; the mayor of Atlantic City solved the controversy by declaring his residents would simply enjoy two meals — Thanksgiving and "Franksgiving." After two years of squabbling (or gobbling, as it were), Congress adopted a resolution in 1941 setting the fourth Thursday of November as the legal holiday.
Read more: http://www.time.com/time/specials/packages/article/0,28804,1862503_1862505_1862523,00.html#ixzz0XzpkJLM5