The Thread
The janai is a cord of spun cotton worn over the left shoulder and under the right arm, received at an initiation ceremony that marks a boy's entry into formal religious responsibility. It is worn continuously thereafter. Once a year, on the full moon of Shrawan, it is renewed: the old thread is cut away and a new one put on, after ritual bathing, a period of fasting, and recitation performed with a priest.
Who Wears It
The rite is not the property of one community. It is kept by Khas Bahun and Chhetri families, by Newar households in Kathmandu, Patan and Bhaktapur who maintain it in their own form, and by Maithili, Bhojpuri and Awadhi households across the Terai — each with its own priestly tradition, its own liturgy, and its own variations in how the day is kept. The strands of the thread are commonly explained as standing for obligations the wearer has taken on, and the explanations differ between traditions.
The Day's Shape
A man renewing his janai typically bathes at a river or a sacred pond in the morning, fasts until the ceremony is complete, and has the new thread placed by a priest with the appropriate recitation. In practice the priest serves many people in sequence through the morning, and the riverbanks and temple tanks are crowded from early on.
Not the Whole Day
It is worth being clear that the janai concerns those who wear it, which is a minority of the population — while the doro tied on the wrist is given to anybody at all who comes for one. The festival is often described as though thread-renewal were its whole content; the wrist thread is what makes the day a shared one.
Related Places & Sites

sacred site
Pashupatinath TempleAmong the sites where the janai is renewed and the doro tied, with priests serving pilgrims through the morning
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