Today is the 15th day of the 10th lunar month, a holiday called Xia Yuan Jie. It’s part of a trio of holidays that include the much more famous Lantern Festival (Shang Yuan Jie, the 15th day of the 1st lunar month) and the Ghost Festival (Zhong Yuan Jie, the 15th day of the 8th lunar month). “Shang” means “upper,” “Zhong” means “middle,” and “Xia” means “down.”
These holidays originated as the festivals of three gods: the Rulers of Heaven, Earth and Water, respectively. The three gods also rule over different aspects of human life: “Tian-Guan [Heaven] gives happiness, Di-Guan [Earth] forgives guilt and Shui-Guan [Water] comes to alleviate dangers.”
Wikipedia says that on this day, people “Set flower shaped lanterns adrift in a stream or river at sundown, give offerings to deceased whose wandering spirits/ghosts may return at night to visit.” Being Wikipedia, no source is provided. The Thailex Travel Encyclopedia seems to corroborate this, as well as providing a theory that many practices more common to the Ghost Festival have been transferred to the third festival of the trio as well:
People also give food offerings to deceased, whose wandering spirits may return at night to visit, a tradition normally associated with Gui Yue [Ghost Month], but it seems that traditions of many a festivals are often mixed. At sundown of this day, people set lotus-shaped lanterns adrift on the water, reminiscent to the Thai festival of Loi Krathong (fig.), and the festival is hence also known as Water Lantern Festival (fig.).
Given that the photograph above is from the Ghost Festival, it would seem that this conflation of practices and festivals is indeed the case. To my mind, there is a certain poetry to making offerings of floating lanterns on the festival of the Ruler of Water, though I admit that’s just UPG.
At any rate, it’s a very beautiful custom whenever it’s celebrated, and I plan to do so tonight.