Five College Center for East Asian Studies Study Tours Website
"Nigatsu-dō was founded by a monk by the name of Sanetada in 752, but the Buddhist monk Jitchu, a pupil of Rōben, later introduced a repentance service dedicated to the image of the eleven-faced Bodhisattva, Kannon in 760. It has taken place as an annual rite since 760 without any break. The service has come to be known as Shuni-e (修二会, lit. Second-Month Service), as it was held in the second month of the traditional lunisolar calendar. At present, it starts on March 1 and ends on the 15th of the month.  Omizutori, which means taking sacred water, has become the popular name of the ceremony.
While the first Shuni-e service is said to have been held by Jichu in another temple in 752, the original construction of Nigatsu-dō hall is estimated to have completed only somewhere between 756 and 772. Nigatsu-dō was destroyed in 1667 due to a fire, and rebuilt in 1669. In 1944, it was chosen by Japan as one of the most important cultural aspects of the country." (Wikipedia)
Wikipedia - Nigatsu-do