The supreme festival in Puri is the Rath yatra or Car Festival, of all the fiestas of the year, which honors the voyage of Krishna from Gokul to Mathura. According to Hindu legends, Krishna, the ninth personification of Vishnu, was the eight son of Basudeva and his wife Devaki. It had been predicted that a son of theirs would slay Kansa, the fiend King of Mathura, who personifies the principal of evil. Kansa, therefore, locked up Basudeva and his wife, and slew their first six sons; Balarama, the seventh, was abstracted from Devaki’s womb, moved to that of Rohini, another wife of Basudeva, and so saved.
After the birth of Krishna, the father runaway from Mathura with the child and, crossing the Jamuna, gave the infant Krishna to the care of the herdsman ruler, Nanda of Vraja. In Gokul Krishna rose up to adulthood. After sometime, Kansa took notice of of him and sent an envoy to carry him and his brother to Mathura. The brothers drove in their chariot gloriously to Mathura, where Krishna slain Kansa and ruled in his stead.
The chapter in the life of Krishna is celebrated by the Rath yatra, which happens in June or July annually. At this time, the descriptions of Jagannath, Balabhadra and Subhadra are detached from the temple and taken in great chariots to the garden house (Gundicha-bari) the length of the Bara Danda road, which is about a mile and a half long. The gods stay in the car at night, and are taken out next dawn and positioned in the temple. They stay there for a week and are then once more placed in to the cars taken back to the holy place, thus observing the come back journey of Krishna. The law is that the entire festival should last 9 days, letting a day for the journey to the Gundicha- bari, a day for the return journey, and 7days for the stay there; but when put into practice it lasts much longer, the return trip every so often taking countless days.
Source – Internet