The Rath Yatra — the Chariot Festival — is one of the oldest and largest religious gatherings anywhere in the world. It takes place annually in June or July, the exact date determined by the lunar calendar (Ashadha Shukla Dwitiya, the second day of the bright fortnight of the month of Ashadha). Up to a million pilgrims descend on Puri in a single day to witness — and participate in pulling — the three massive wooden chariots that carry Lord Jagannath, his brother Balabhadra, and his sister Subhadra from the main temple to the Gundicha Temple 3 kilometres away along the Grand Road. The chariots are pulled by thousands of devotees on thick ropes; the enormous crowds make progress slow, stop-and-start, and extraordinarily loud.
The chariots themselves are architectural achievements rebuilt fresh each year from new wood, taking teams of craftsmen months to construct. Lord Jagannath's chariot, Nandighosa, is the largest — typically around 14 metres tall with 16 wheels, covered in yellow and red cloth, its roof conical and layered like a smaller version of the main temple tower. Balabhadra's chariot Taladhwaja is blue and red; Subhadra's Darpadalana is black and red. Each is distinct and recognisable from far away. Once the procession has deposited the deities at Gundicha Temple, they remain there for a week before the return journey (Bahuda Yatra), which is in many ways as well-attended as the outward procession.
If you plan to attend Rath Yatra, start organising months in advance. Accommodation in Puri fills completely, as does everything within a 50km radius. The entire city becomes a single event for the duration. The crowd on the Grand Road during the procession is dense to the point of being physically challenging; wear minimal valuables, stay aware of crowd pressure, and position yourself early. What you will witness is something that has happened in essentially the same form for over 800 years — the same road, the same chariots, the same deities making the same journey, surrounded by the same devotion scaled up to the size of a city.