At this festival the snowy range of Kanchendzonga is worshipped for its unifying powers. This festival also marks the signing of the treaty of brotherhood between the Lepchas and Bhutias. The local deities were invoked to witness the historic occasion. The guardian deity is portrayed by a masked Lama dancer as a fiery red-faced deity wearing a crown of five skulls and riding a snow-lion, a truly colourful spectacle indeed.

Bum - means "Pot or Vase" Chu - means "Water. The pot containing the Holy water is opened by the Lamas of the monastery. The level of water in the pot tells of things to come in the forthcoming year. If the water is to the brim, it prophesies bloodshed and disturbances and if the pot is almost dry it signifies famine. However, if it is half full it foretells a year in which peace and prosperity will reign. A part of the holy water is distributed amongst the participants and then the pot is replenished with river water and sealed at the end of the festival to be opened only in the next Bumchu.

This festival symbolises the descent of Lord Buddha from the heavens. Lha - means "Heaven" Bab - means "Descent". Legend has it that when Lord Buddha attained Enlightenment, through his spiritual powers he got to know of the whereabouts of his mother and at the age of forty one ascended to the heavens along with thousands of his followers. He stayed in heaven for three months where he preached to his mother but other celestial. Maugalyayana, Buddha's disciple who was on earth as his representative and who also possessed miraculous powers went up to heaven to ask the Lord to return back to the earth. The gods were unwilling to let Lord Buddha return to earth so Maugalyanana suggested that the celestial beings could come to earth instead to attend his preaching. So Lord Buddha decided to come back to earth at a place called Sankasya along a triple ladder that was prepared especially for the occasion by Viswakarma, the God of Machines. This festival hence celebrates this legend of the Lord Buddha.

This festival celebrates Buddha's first preaching of the four "Noble Truths" to his first five disciples in Deer Park at Sarnath. At Gangtok, there are prayers at the Deer Park and at a secluded place called Muguthang in extreme North Sikkim, followed by a Yak race.