What is Thaa Yai?
Thaa Yai is a yearly event when Thai people go to their local temple to remember the deceased members of their family. The religious holiday is celebrated at different times of the year in different parts of Thailand, and also called various names. In Koh Samui they use the Southern Thai phrase 'Thaa Yai' to describe this event.
In Koh Samui Thaa Yai is celebrated in September. Traditionally, people prepare a dish and bring it to the temple. All the dishes are put together to make a feast. At the same time offerings of food are made to monks at the temple.
Thaa Yai, Obon and Buddhism
It is believed that the dead come back during Thaa Yai. The holiday shows reverence for dead family members. In this respect Thaa Yai is very similar to Obon in Japan that also celebrates the return of dead family members. Obon varies from region to region in Japan but usually occurs in August.
With both Obon and Thaa Yai these festivals are nominally Buddhist, but probably pre-date Buddhism since one of the central tenets of Buddhism is re-incarnation. There are no 'ghosts' of the dead since they have already started a new life in a new incarnation. Rather the ceremony is probably based in the idea of Ancestor Worship which comes from China.
In Thaa Yai it is believed that the dead return for 2 weeks before they go back to whence they came. The food offered to the dead has wooden skewers stuck in it. On these skewers are messages for the dead. During the 2 weeks of Thaa Yai people go regularly to their temple to give offerings to the dead and to monks. The end of the two week period is marked by a final big feast.
Internet and Thai Culture
Finally, it is worth mentioning that very little is written about this festival. It is only Camille's Samui Blog that notes the festival. The bigger point is that despite the information age in which we live, there is very little documentation about Thai culture.