I read that it was proclaimed as a holiday by Lincoln when he heard that Sherman had reached the sea (Charleston I believe). Is this true?
In the past, presidents would declare days of thanksgiving from time to mark particular events. The last ones I know of were days of thanksgiving to recognize victory over Germany and Japan at the end of World War 2.
These specific days of thanksgiving are not to be confused with the annual November Thanksgiving holiday. This was inspired by the Pigrims and turned into a national tradition due to the lobbying of a magazine editor during the 1830s (tho’ not made a legal holiday till 1941):
Oh, and Lincoln was indeed the first president to recognize the November Thanksgiving with an official proclamation in 1863 (though not a day off):
http://members.aol.com/calebj/proclamation.html
Note that no specific battle or event is mentioned.
The creation of Thanksgiving as a national holiday was promoted by the editors of Godey’s Lady Book magazine. It had been a fairly popular regional holiday in New England.
The master speaks:
http://www.straightdope.com/classics/a2_147.html