A nagar kirtan is a procession through a town or village with the Guru Granth Sahib Ji carried on a decorated float, led by five Sikhs representing the Panj Piare, with kirtan sung along the route and gatka displayed.
The route is swept and washed ahead of the procession by residents, and stalls set up along it give out food and drink to anyone passing. Those stalls are put up by households, not by the gurdwara, and there is a strong element of competition in it.
It is the main public form of Sikh practice outside the gurdwara and is held on gurpurabs and before major festivals.
The largest in the world outside Punjab are at Yuba City in California and at Surrey in British Columbia, each drawing well over a hundred thousand people, and both are on scales that no Punjab town matches for a single procession.
Everything given out along the route is free and nobody asks who anybody is. That is the whole point of it.
The five walk barefoot ahead of the float and the road in front of them is washed as they go, so the procession moves at the pace of the people cleaning it rather than at the pace of the traffic.
It is held for the Gurpurabs and for local anniversaries, and in the diaspora it is the one occasion on which the community appears in public at scale.
The Vaisakhi nagar kirtans in Surrey and in Southall draw hundreds of thousands. They are the largest Sikh gatherings held anywhere outside India.