Chasok Tangnam is the most important festival of Limbu among other festivals. This festival falls on a full moon day of the month of Senchengla or the Mangsir month of Nepali calendar. "Tangnam" simply means festival in Limbu language. Along with the resurgence of Limbu identity, culture and nationalism, Limbus have started to celebrate Chasok Tangnam as an important festival of their calendar. Government of Nepal has allocated three days of Mangsir as Kirat Festival for Limbus to celebrate Chasok Tangnam. Expatriate Limbus in Hong Kong, United Kingdom, USA, UAE, Qatar, KSA, Canada, Korea and in many countries also celebrate Chasok Tangnam.
Traditionally, Limbus celebrate the Chasok Tangnam festival at home with the first harvest being offered to Yuma Sammang (Limbu ancestor goddess) and other deities. They worship and thank Yuma, God Theba, and other deities for a successful harvest of the year. The festival wasn't celebrated in a social group until recently; traditionally Chasok worshipping is done at home by individual family and specially by the female member of the household who has borne a child. Limbus conduct Chasok worshipping according to their will and scheduled throughout the month of Senchengla / Mangsir and not on any specific date. Now majority of Limbus try to finish or conduct their Chasok worshipping on the full moon's day.
Limbus like other Kirat people are agrarian. They are also Shamanistic in religious practices. Nature worshipping is the main principle in Kirat religion. Chasok Tangnam developed as a festival among Kirat Limbu people to thank the mother nature for harvest and their ancestors for handing down the teachings of agriculture.
During the ceremony, Limbu priest also recites the story of the Limbu ancestors who endeavoured hardships leading the life of hunting and gathering. Then, they were taught by the spirit of Yuma Sammang to plant and harvest various crops. When the crops were harvested Limbus decided to place the little portion of harvest as offerings to their supreme God Tagera Ningwaphumang and the ancestor Goddess Yuma Sammang for the knowledge of agriculture and harvest. It is strongly and widely believed that any harvest eaten without first performing Chasok Worship and without offering to the Gods is cursed with badluck and misfortune.
Kirat Rai people celebrate this festival as Sakela Udhauli. Kirat Yakkha, Sunuwar and other Kirati people celebrate their own version of this festival at the same time as Limbus with their own name and tradition. The Major theme of this festival is worshipping of the ancestors, gods and nature for the year's harvest.