Boon Khao Pradap Din (บุญข้าวประดับดิน) is Month #9 of Heet Sip-Song – The Twelve Isan Merit-Making Traditions.
Khao Pradap Din means "food placed on the ground" and it's a ceremony to honor people's ancestors. Before dawn (usually, though some places do it in the daytime), people take foods like sticky rice, fruit, fish, and sweets, plus betel and cigarettes, all served on or wrapped in banana leaves, to the temple. Usually these packages are laid on the ground (hence the name) along the boundary wall or under a tree, though sometimes the food is put elsewhere such as atop the boundary wall, on a ledge around the ubosot, or hanging in a tree. It is believed that spirits are able to roam to earth at this particular time and so they are invited to come eat on their arrival. Wandering ghosts who don't have any relatives living in the area are also welcome to come join. The food is usually placed outside the temple because it's believed that spirits cannot go onto temple grounds; however, some places believe that everyone, even ghosts, can visit temples and so the food is given inside the temple walls.
One original part of the ceremony – paying respect to the spirits of the land that allow people to grow crops – used to follow the feeding of the ancestors, but it is no longer widely practiced as it is also part of Boon Khao Sak which happens two weeks later.
Later in the day people will offer a special meal for the monks and then do the standard gruat nam water ceremony to pass on merit to their ancestors. Although its origin is pre-Buddhist, Boon Khao Pradap Din, or just Boon Pradap Din as it's often called, is now associated with King Bimbisara, who after converting to Buddhism was taught by the Buddha the importance of dedicating merit to ancestors.
Boon Khao Pradap Din happens during the new moon on the last day of the lunar month.