It is a Buddhist celebration centred around the That Luang, the name of the oldest and most highly acclaimed of all Buddhist temples (stupas) in Laos.
The That Luang Festival occurs for three consecutive days, beginning on the full moon of the twelfth month of the Buddhist Calendar. This will generally put the festival sometime in November on the Gregorian Calendar.
The That Luang stupa was originally a Hindu temple, built in the First Century A.D. By Third Century A.D. it became a Buddhist temple. It had to be rebuilt several times throughout its long history, and the modern building was built in 1566.
Inside the That Luang is believed to be certain relics of Buddha. The temple is covered in gold and sits in the middle of Vientiane, the capital city of Laos.
During That Luang Festival, Buddhist monks gather at the stupa for religious ceremonies. They make offerings, offer incense, circle the stupa thrice, and do other religious rituals meant to earn merit. There are also musical and dramatic presentations at the festival, and both people and monks participate.