The Dish
Kwati is a thick soup made from nine varieties of bean and pulse, sprouted and then cooked together with ginger, garlic, cumin, ajwain and spices, often with meat or bamboo shoot. Which nine is not fixed — the set varies by household, by community and by what the market has, and published lists disagree with each other and with what any given family actually cooks. It is eaten with rice and is substantial to the point of heaviness.
Once a Year
Kwati is cooked on Janai Purnima and, in most households, essentially not otherwise. The constraint is built into the dish: the beans must be soaked and left to sprout over several days before they can be cooked, so preparation begins well before the festival, and the soaking bowl on the kitchen shelf is itself the announcement that the day is coming. Mixed sprouting beans appear in the markets in the run-up, sold ready-measured for the purpose.
Why Sprouted Beans
The explanation usually given is seasonal and medical rather than symbolic. Shrawan falls at the height of the monsoon, a period associated with damp, sluggishness and illness, and the sprouted beans are held to be warming, strengthening and easier to digest than dried pulses — a deliberate dietary intervention at the year's least healthy point. Whatever one makes of the reasoning, the timing is deliberate and the dish is firmly tied to it.
Gunhu Punhi
Newars know the day as Gunhu Punhi, or Kwati Punhi after the food, and it falls inside Gunla, the holy month observed by Newar Buddhists with dawn processions and music. The food, the thread and the month overlap on this one date, which is a good deal of the reason the day carries the weight it does.
Eating It
Kwati is served in Newar restaurants across Kathmandu and Patan around the festival and is worth seeking out then, since it largely disappears afterwards. It is also the dish most likely to be pressed on a visitor invited into a home on the day.
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