Not a national holiday, but a time when Thais pay respect to the goddess of the waters by floating candlelit offerings on any and all waterways around the kingdom. 
Loy Krathong, the name comes from Loy to float and Krathong the vessel made from banana leaves that is floated is thought to originate from Sukhothai although some scholars believe its origins of from Bangkok.
It is a lunar Festival and therefore not fixed. It usually falls in the month of November.
Most activities take place in the evening with celebrations, parties, parades and festivals as well as the giving of thanks with the floating of the Krathong.
In ChiangMai and the North of Thailand (Lanna) Loy Krathong coincides with the Lanna Festival of Yi Peng at which time the people of the region send many lanterns known as khom loi into the sky.
Yi Peng (Loy Krathong) is a wonderful spectacle in ChiangMai with parades of Festival queens and princesses from surrounding villages towns and cities, music fireworks entertainment and thousands of lanterns. 

Includes some from a village's celebration. It isn't just the big cities that hold events and parades, you will find events in many small towns and villages usually leading up to Loy Krathong itself so that winning Queens and Princesses can join the city parade.

As always we welcome your input and you can contribute via the Contact Al page, the image left is a link.
Whatever your (Thai) information needs we hope this site is of help and if you think of anything to include just let us know.