Chaturdashi is the fourteenth tithi, falling on the eve of the full moon in the bright fortnight and the eve of the new moon in the dark. In the fivefold grouping it is a Rikta tithi, classed as generally unfavourable, and it carries a strong association with Shiva in his fierce forms: the dark-fortnight Chaturdashi of the month of Magha or Phalguna is Maha Shivaratri, the great night of Shiva, one of the most important Shaiva festivals of the year. Naraka Chaturdashi, in the dark fortnight around Diwali, is another well-known observance. The waxing Moon at the bright Chaturdashi is all but full. The panchanga treats the tithi with caution for ordinary matters while marking its powerful nocturnal festivals. These belong to the calendar and devotional tradition.
Maha Shivaratri's date, unlike most major festivals which are fixed to a tithi at sunrise or midday, is determined by which Chaturdashi prevails during the nishita kala, the deepest part of the night, typically the period around midnight, reflecting the festival's explicitly nocturnal character; a panchanga calculating the date must therefore identify the tithi in force at that specific hour rather than at any daytime reference point. Naraka Chaturdashi, by contrast, is fixed to a pre-dawn bathing ritual associated with Krishna's slaying of the demon Narakasura, and the two Chaturdashi observances, one at midnight and one before dawn, illustrate how the same numbered tithi can host festivals anchored to quite different hours of its roughly one-day span.