Significance
Dhanteras, or Dhanatrayodashi, opens the five-day Diwali festival. It honours Lord Dhanvantari, the divine physician who emerged from the ocean churning with the pot of amrita, and is dedicated to health, wealth and prosperity.
Buying gold, silver and new utensils on this day is believed to invite Goddess Lakshmi's blessings.
Rituals
Homes are cleaned and doorways decorated with rangoli and footprints of Lakshmi. People purchase precious metals and utensils, worship Dhanvantari, Lakshmi and Kubera in the evening pradosh muhurat, and light the first lamps and a Yama-deepa to ward off untimely death.
How the date is fixed
Dhanteras falls on the Trayodashi of Kartika Krishna paksha, decided by the evening (pradosh) presence of the tithi, so the pradosh muhurat is shown above for your location.