The Belief
The Kumbheshwar Temple in Patan — a rare five-storey pagoda, and one of the oldest standing structures in the Valley — has a sacred tank in its courtyard. The water in it is held to come underground from Gosaikunda, the high alpine lake sacred to Shiva at around 4,380 metres in the Langtang range. A jar of water is brought down from the lake and added to the tank for the festival.
Why That Matters
Gosaikunda is a genuinely hard pilgrimage: multi-day, high-altitude, seasonal, and beyond the reach of the old, the unwell, the poor and the busy. The belief about Kumbheshwar's water makes the merit of that pilgrimage available in the middle of a city to anyone who can walk to Patan. This is a substitution performed openly and without embarrassment — the festival's most practical piece of theology, and the reason the courtyard is packed from before dawn.
The Crowd
Thousands come to bathe in and around the tank, to receive the doro, and to make offerings. The courtyard is dense, wet and slow-moving for most of the day, and the surrounding lanes are full of stalls. It is one of the Valley's great crowd scenes and not a place to bring anything you mind losing.
The Sadhus
Sadhus gather at Kumbheshwar in numbers matched at few other Valley festivals outside Shivaratri — ash-smeared, dreadlocked renunciants, some resident in Nepal and some travelling from India for the day. They sit around the tank and the temple platforms receiving visitors and offerings. Photographing them is often invited and then charged for; agree on that before the picture rather than after.
Those Who Do Go
Pilgrims do still make the walk to Gosaikunda itself for the full moon, and the lake shore at that altitude on that night is a different festival entirely — cold, thin-aired, and far smaller.
Related Places & Sites

sacred site
Kumbheshwar TempleThe five-storey temple in Patan whose tank is the focus of the day; held to be fed from Gosaikunda

destination
GosaikundaThe high lake in the Langtang range the tank is believed connected to, and the pilgrimage Kumbheshwar substitutes for
Related
Connected places and entries worth pairing

Gosaikunda
Gosaikunda is the major high-altitude lake pilgrimage for Janai Purnima

Panch Pokhari
Panch Pokhari also draws Janai Purnima pilgrims

Kumbheshwar Temple
The tank at Kumbheshwar is the focal point of the day in Patan

Kalinchowk Bhagwati
The Dolakha summit shrine and kunda where the full moon draws its own bathing pilgrimage

Gunla
The day is the full moon of the Newar Buddhist holy month

Gai Jatra
Sa Paru follows the next day

Nag Panchami
Serpent worship ten days earlier, in the same Shrawan cluster
Part of Janai Purnima