The Day of Passion is a sacred holiday associated with the goddess Irashi. On this day, worshipers of Irashi must exert themselves to the point of exhaustion. It is thought that if you die from your work on this day, you will be able to serve the goddess forever in your afterlife.
Outside of the worship of Irashi, many view this as a sort of date night, and it is a popular day for consummating relationships.