The modern incarnation of Thanksgiving was founded during the middle of the Civil War, by Abraham Lincoln, who declared it a national holiday in the hopes of boosting the war-torn Union's morale. It's a reminder to be thankful for a bountiful harvest and all the good things that are still functioning, despite the pains caused by a rightful and just struggle.

There are earlier thanks giving celebrations in the US's history (it is an imported Puritan tradition), but that's the one where it became a permanent national holiday, and it's also the cool one.