Overview
Dhanteras (Hindi: धनतेरस), also known as Dhanatrayodashi (Sanskrit: धनत्रयोदशी), is the first day that marks the festival of Diwali in most of India, and Tihar in Nepal. The festival is observed on the thirteenth lunar day (Trayodashi) of Krishna Paksha, the dark fortnight, in the Hindu calendar month of Ashwin according to the amānta tradition, or Kartika according to the Bikram Sambat tradition.
The day is associated with the worship of Dhanvantari, who is regarded in Hindu tradition as the god of Ayurveda. According to these traditions, Dhanvantari imparted the wisdom of Ayurveda for the betterment of mankind and to help relieve humanity from the suffering of disease. Owing to this association, Dhanteras carries a special significance for practitioners and followers of Ayurveda, alongside its wider cultural role as the opening day of the Diwali festivities.
In recognition of the day's connection with Dhanvantari and Ayurveda, the Indian Ministry of Ayurveda, Yoga and Naturopathy, Unani, Siddha and Homeopathy (AYUSH) announced its decision to observe Dhanteras as the "National Ayurveda Day". The first National Ayurveda Day was observed on 28 October 2016, coinciding with Dhanteras that year.
As the inaugural day of the Diwali sequence, Dhanteras is followed by the other days of the festival observed across various regions of India and Nepal. Its placement on Trayodashi of Krishna Paksha situates it within the broader lunar reckoning that governs the timing of Diwali and Tihar in the Hindu calendar.
References
Adapted from the English Wikipedia article on Dhanteras.