Krishna Chaturdashi and the Great Night of Shiva

Krishna Chaturdashi is the twenty-ninth tithi overall, the fourteenth day of the dark fortnight, falling on the eve of the new moon when the Moon has all but disappeared. In the fivefold grouping it is a Rikta tithi. It carries the most powerful of the Shiva observances: Maha Shivaratri, the great night of Shiva, is kept on the dark-fortnight Chaturdashi of Phalguna, observed with night-long fasting and vigil. The monthly dark Chaturdashi, Masik Shivaratri, is likewise devoted to Shiva. The waning Moon is now invisible against the Sun's glare. The panchanga treats the tithi with caution for ordinary matters while marking its nocturnal Shaiva festivals as among the most sacred of the year. These belong to the devotional calendar.

Because Maha Shivaratri is calculated by the tithi prevailing during the nishita kala at the heart of the night rather than by the sunrise convention used for most calendar dates, the festival can fall on either of two consecutive Gregorian calendar dates depending on the year and one's time zone, which is why different regional panchangas occasionally publish different dates for the same observance in the same year, a genuine and openly acknowledged point of calendrical disagreement rather than an error on either side. The monthly Masik Shivaratri, observed on every dark-fortnight Chaturdashi rather than only the one in Phalguna or Magha, follows the same midnight-tithi calculation on a smaller, non-festival scale twelve times a year.