The holiday's origins aren't made clear in the episode, showing that much like the real Thanksgiving, it celebrates the arrival of the Dispilgrims on their boat, the Maysour. It also seems to celebrate the settlement between the native slugs and worms.
The traditions on the holiday include decorating bare sticks with garbage, eating hot bowls of glob and the gathering of Grouch families, who then complain about each other. They also sing the traditional Trashgiving carol. There is also the annual Trashgiving Day Parade, featuring music, clowns, floats and balloons.