The story
Jagannātha, lord of the world, is Kṛṣṇa in the form Puri worships: a wooden figure with round eyes and stumps for arms, beside his brother Balabhadra and his sister Subhadrā. Once a year the three leave the sanctum and are pulled two miles down the Grand Road to the Guṇḍicā temple, where they stay a week before returning.
The reason given is that Subhadrā asked to see the city, and her brothers took her. Another telling makes the Guṇḍicā temple the aunt's house, and the journey a family visit.
The point of the day is access. A temple that admits only Hindus puts its deities in the open street, where anyone at all may see them and pull the rope. The English word juggernaut comes from the chariots and the crowds that surrounded them, and is a traveller's misreading of the day rather than a description of it.
