Krishna Trayodashi and the Pradosha of the Dark Half

Krishna Trayodashi is the twenty-eighth tithi overall, the thirteenth day of the waning fortnight. Like the bright-fortnight thirteenth it carries the Pradosha vrata, the twilight observance dedicated to Shiva, so the fast recurs on the thirteenth tithi of both halves of the month. In the fivefold grouping it is a Jaya tithi. The Dhanteras observance that opens the Diwali season falls on the dark-fortnight Trayodashi of Kartik. The waning Moon is now a hairline crescent near the Sun, soon to vanish. The panchanga reads the thirteenth tithi as generally auspicious, carrying a strong Shaiva current through the Pradosha vow. These are the conventions of the calendar and festival tradition.

Dhanteras, opening the multi-day Diwali observance on the dark-fortnight Trayodashi of Kartik, is dedicated to Dhanvantari, the physician-god who is said to have emerged from the churning of the cosmic ocean bearing the nectar of immortality, and the day's strong association with purchasing gold and new utensils draws on his identity as a bringer of wellbeing and prosperity rather than on the tithi's own Pradosha character, which continues to apply independently on this date as it does on every thirteenth tithi of either fortnight. The convergence of Dhanteras and Pradosha on the same waning Trayodashi in Kartik is a case of two separately timed observances, one keyed to the specific month and one recurring every fortnight regardless of month, falling together.