Many people are of the mistaken belief that Dhanteras is about the celebration of wealth. Dhan is the short form for Dhanvantari, God of health and wellbeing and teras is the thirteenth day after the full moon. It is actually the worship of Dhanvantari that is done on this day as a part of the Deepavali celebrations.
Dhanvantari is the manifestation of Bhagavan Vishnu. His first manifestation was during the samudra-manthan, the churning of the milky ocean by the deva-s and asura-s for getting amrtam. Worship of Dhanvantari is the worship of Vishnu.
One offers yellow flowers and does decorations in yellow. One can offer anything as prasada. But there is also a custom of eating triphala as prasada. Triphala (the three fruits) is an Ayurvedic preparation comprising bibhitaki, amalaki, and haritaki, all native to India. The innumerable benefits of triphala for general health and wellbeing are recognised by Ayurveda, which is the ancient healing system of medicine and well-being that has helped a living civilization for more than 6000 years. In some pujas of Dhanavantari, triphala is given as prasada.
After all, he is the healer. What better prasada can one get from a healer than something that can heal? If you go to any traditional Ayurvedic hospital, there will always be a small shrine for Dhanvantari on the premises, taking Vaastu into account. Therefore, on the thirteenth day after the full moon, the day before Deepavali, Dhanvantari is to be worshipped. I cannot help but speak here about how some people are upset about the ban on crackers.
It is important to note that Diwali is really Deepavali, a row of deepa-s, lights. Crackers came into the picture only a couple of hundred years ago, when chemicals for explosions became more readily available. Hindus need not get upset about the cracker ban but enjoy the lighting of diya-s for this Deepavali.
The day after Dhanteras is Naraka Chaturdashi or Kali Chaudas (in North India) when Narakasura was killed by Krishna. Deepavali involves a ritualistic bath signifying purification followed by Pratipada. In the business community, there is pustak pujan. Bhaiya Dooj is also celebrated, more in North India.