Before the 17th century, Buddhist discipline required monks to stay in their temples for weeks in the summer. This prevented them stepping on small creatures outside for a time, and taught them to live in harmony with one another. When finally allowed to leave their confinement, monks went down to the mountains, where laymen would prepare yogurt for them as alms. The monks enjoyed the yogurt and happily celebrated their newfound freedom.
This is the origin of the "Xuedun" or Shoton Festival at Drepung, which takes place every August. Today, the Shoton Festival is a time for monks to go the mountains for contemplation, after which time their families will meet them on the mountainside. Many lay Buddhists make a pilgrimage to Drepung during this time and participate in the festivities, which include performances by the Tibetan Opera.
The Shoton Festival begins with the dramatic unfurling of a giant thangka banner of the Buddha, amidst incense smoke, the sound of bugles, and scripture recitations. Devotees rush to make offerings before it is rolled up again in less than two hours.