The Great Emperor Water Official(Image: Bravotaiwan.com.tw)
The full title of this Taoist deity is The Great Emperor Water Official of Pervasive Yin of Lower Primordial & Tertiary Grade Who Eliminates Distress (下元三品解厄水官洞陰大帝 Xiayuan Sanpin Jie E Shuiguan Dongyin Dadi), so that is why the day is also known as the Xiayuan (Lower Primordial) Festival.
This deity is part of a Taoist trinity called the Three Great Emperor Officials (三官大帝 Sanguan Dadi) which includes the Heaven Official (天官 Tianguan) (also mentioned here), Earth Official (地官 Diguan) (also mentioned here), and Water Official (水官 Shuiguan). The Great Emperor Water Official embodies the vital energies of wind and water and that of the vast dawn. He is in charge of all deities that reside in or are related to water.
It is believed that on this day, the Water Official descends to the earthly realm to inspect and assess the sins of mortals and to eliminate their misfortune and distress if he sees it is fit to do so. On this day, Taoist followers hold ceremonies in honor of the Great Emperor Water Official with the common theme of praying for fortune and averting disaster.