This festival is known as 'Monlam Chemo' in Tibetan. It falls on 4th-11th day of the 1st Tibetan month in Tibetan Buddhism. The Great Prayer Festival was established by the Great Saint Lama Tsong Khapa (also known as Losang Drakpa) in 1409in Lhasa. All Tibet was invited to a two-week-long festival of prayer, auspicious rituals, teachings, and celebrations, from the first new moon until the full moon of the lunar New Year. Many hundreds of thousands, perhaps more than a million, came from near and far. As the greatest religious festival in Tibet, thousands of monks, of the three main monasteries of Drepung, Sera and Ganden, gather for chanting prayers and performing religious rituals at the Jokhng Temple in Lhasa.
That time of the year was chosen because Lama Tsong Khapa firmly believed in the life story of the founder Guru Shakyamuni Buddha, told in the Indian Buddhist Sutra of the Wise and the Foolish called "Overcoming the Six Teachers: "Buddha was challenged by six rival teachers to a contest of miraculous performances. For many years, Buddha evaded their challenges, letting people believe that he was afraid of their magical powers, losing his royal patrons, and causing doubts and worries to grow among the people.
It is said that during that first Great Prayer Festival in 1409, all the people who gathered in Lhasa had visions of Buddhas and divine beings filling the sky. Everyone got into their most religious mood and spent the whole time as if on a spiritual retreat, praying, studying, making offerings, teaching, learning, and debating meaningful philosophical topics.
The noble tradition is preserved and is practiced in the same way to this day in most of the Gelug monasteries. On the full moon, the Day of Miracles is the most special day of the festival, when thousands of people, lay and ordained alike, come to pray, view the big butter sculptures, and make offerings to the Sangha.
The main purpose for the Festival is to pray for the long life of all the holy Gurus of all traditions, for the survival and spreading of the Dharma in the minds of all sentient beings, and for world peace. The communal prayers, offered with strong faith and devotion, help to overcome obstacles to peace and generate conducive conditions for everyone to live in harmony.
Traditonally, from the New Year's Day until the end of the Monlam, lay Tibetans would make merry. Pilgrims from all over Tibet would join the prayers, and make donations to the monks and nuns. Many other monasteries would hold special prayer sessions and perform religious rituals.