In Vedic timekeeping, a tithi is a "duration of two faces of moon that is observed from earth", known as milа̄lyа̄ in Nepal Bhasa, or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. From Wikipedia
The festival marks the culmination of Chaitra Navratri with rituals, fasting, and processions, centered in Ayodhya, the birthplace of Lord Rama.
