The kanwar is a pole carried across the shoulders with a vessel of Ganges water hanging at each end, and the rule that defines the observance is that from the moment it is filled until the water is poured over the lingam, the kanwar must not be set down on the ground. That single prohibition turns a long walk into a continuous discipline: walkers rest by hanging the pole from a tree or a purpose built stand, they eat and sleep in relays, and the strictest form, the dak kanwar, is not rested at all but run in relay by a team until the water is delivered.
The water is drawn at a source of the Ganges. Most take it from Haridwar, some from Gaumukh or Gangotri, and those bound for Baidyanath at Deoghar take it from Sultanganj in Bihar and walk something over a hundred kilometres. The month is Shravan, in July and August, which is the monsoon, and the walkers wear saffron.
The story behind it is the churning of the ocean. When the poison halahala surfaced and threatened everything, Shiva drank it and held it in his throat, which turned blue, and the offering of cool river water over the lingam in the hot months is understood as continuing to relieve that burden.
The scale is modern. A local practice of limited size grew from around the 1980s into one of the largest annual movements of people in India, with highways closed for weeks, medical camps, sound systems mounted on trucks and a substantial policing operation. The growth is documented; the reasons for it are argued over, with better transport, urban migration and organised sponsorship all proposed.
The vessels are commonly decorated, and the whole assembly is treated as consecrated for the duration.