Once upon a time...
Lord Krishna's evil uncle, Kamsa, tried countless tactics to kill him over the years. In yet another failed ploy to have Krishna and his older brother, Balarama, killed, Kamsa invited them to Mathura for a wrestling match. A chariot was sent to chauffer the brothers from Gokul. Devotees celebrate this day of departure as Ratha Yatra.
Ratha Yatra, or the Festival of Chariots, originated in the 12th century in Indian holy city of Jagannath Puri. The Festival celebrates Lord Krishna’s return to Vrindavan and signifies the Lord’s love for His devotees.
On the day of the festival, Lord Jagannatha -- believed to be an avatar of Vishnu -- is honored in the form of carved wooden stump with large facial features, but no arms or legs. Lord Jagannatha's siblings, Lord Balabhadra and Subhadra Devj, are also celebrated alongside.
... But why the odd wooden form?
"Krishna's grief-stuck siblings, Balabhadra and Subhadra, rushed into the Dwarka sea carrying his half-cremated body. At the same moment, King Indrayumna dreamed that Krishna's body had floated back up on his shores as a log. Indrayumna decided to build a temple to house the log. Legends say that Vishwakarma, God's own architect, arrived as an old carpenter. He agreed to carve the idols, but on the condition that he was not to be disturbed. However, when he did not emerge from his workshop for weeks, going without food, water or rest, a worried and impatient King threw the door open. At the time the images were only half-finished, but the carpenter disappeared. Still, believing the idols to be made from the very body of God, the King sanctified them and and placed them in the temple."
On the day of the festival, the wooden deities (which are usually kept in the Jagannath temple) are each placed in massive chariots that appear like temples. This is the only festival in the world where deities are taken out of temples to travel to devotees, and it is also the largest chariot procession in the world.
Devotees then pull these chariots to the neighboring Gundicha temple 2km away, which is the birthplace of Lord Jagannatha. The statues are then left at Gundicha temple for 7 days in order to let the Gods rest before they are returned to the original temple.