GOSHOKI HOONKO - 御正忌報恩講 - "honorific-death-anniversary-repay-indebtedness-observance":

DEATH MEMORIAL OF SHINRAN SHONIN JANUARY 16th

Hoonko, meaning a gathering to express our indebtedness and gratitude, is a memorial service in memory of Shinran Shonin, and is an occasion to express our gratitude to Amida Buddha for having awakened us to life's supreme meaning. Hoonko is the most important Jodoshinshu observance in Japan. At the Honzan (本山 -"main-mountain") or mother temple in Kyoto, Hoonko is a week-long observance from the 9th through the 16th of January. 

According to the western calendar, Shinran Shonin was born on May 21st 1173 and died on January 16th 1263. According to the traditional lunar calendar, this memorial service began with Shinran's disciples gathering on the 28th day of each month in memory of their master. The 3rd Gomonshu (御門主 - honorific-gate-master") of the Hongwanji, Kakunyo ( 覚如) some 33 years later, set the regulations for this ceremony called Hoonkō Shiki. Later, the 4th Gomonshu, Zonkaku (存覚) set the observance for once a year. The ceremonies were started on the 21st and concluded on the 28th of the 11th month according to the lunar calendar. In 1872, the Hongwanji-ha (Nishi Hongwanji) changed to the western calendar, setting the date at January 16th. The Otani-ha (Higashi Honganji) observes Hoonkō according to the old lunar calendar but has set the day as the 28th of November. 

Hoonkō is one of the principal holidays in which one participates in the art of "hearing the light" through listening to sermons, talks, ritual, and Sangha fellowship.