The ceremonies (ngan) at Prasat Phanom Rung can be split up in two groups: The traditional ones and the modern ones.
The main traditional ngan took place every year at full moon in the 5th lunar month, April, when the locals ascended the mountain from all nearby villages and districts to perform Hinayana Buddhist rites followed by musical shows (mawlam) and dance. The participants were the local ethnic Khmer and Lao farmers and this traditional festival is still the one the elder people remember with most reverence.
The full moon festival did not coincide with the days when the rising sun is visible through 15 gates the temple. The full moon in April falls on different dates in succeeding years, but the solar event fall on the same date in the beginning of April – some years differing 1 day.
Ascending mountains for merit-making ngan at the full moon in April is not a phenomenon limited to Phanom Rung. At many other ancient Khmer sites in Isan the locals choose the same day for performing their traditional ceremonies - alternatively at the Water Festival (songkran) starting 13 April.
Other rituals of a more private character are conducted throughout the year, mostly in connection with full moons. The author has often met traces of nightly ceremonies, when arriving early morning for solar observations.