Dhanateras

The first day of this festival is "Dhanatrayodashi or Dhanteras". It is not about "Dhan" (wealth) as the name apparently suggests but in fact it is celebrated in remembrance of the God of health "Dhanvantari" who came out of the ocean as a result of churning of ocean i.e "Samudramanthan".

Another Grandma's story about Dhanteras goes like this:

Once the King of Indraprastha, Hansraj, went for hunting. On his way to the jungle, he was welcomed by another King of small town, Hem. While king Hansraj was at King Hem's palace, Hem's wife gave birth to a baby boy. This boy had a destiny that on 4th day after his maarige he would die by means of snake-bite. Now Hansraj, pleased by Hem's hospitality decided to save Prince's life. So he built a house under water in the Yamuna river. The Prince was safe there for the time being. But who can deny destiny? When he became 16 years old, he married to a beautiful princess and and per his destiny he died on 4th day after his marriage by snake bite. Everybody was very sad on the prince's death. They  prayed to the God of Death (Yama) and asked him that nobody should suffer like the Prince did. Yama felt bad about the sorrow that descended upon the Prince's family and  said that those who will celebrate the Diwali with all the Diyas lit, will never have to undergo this pain.

Another variation of the same story goes in a different manner and has an happy ending.

Once the sixteen year old son of King Hima was doomed to die by a snake-bite on the fourth day of his marriage as per his horoscope. On that particular fourth day of his marriage his young wife did not allow him to sleep. She laid all the ornaments and lots of gold and silver coins in a big heap at the entrance of her husband's boudoir and lighted innumerable lamps all over the place. And she went on telling stories and singing songs.

When Yama, the god of Death arrived there in the guise of a Serpent his eyes got blinded by that dazzle of those brilliant lights and he could not enter the Prince's chamber. So he climbed on top of the heap of the ornaments and coins and sat there whole night listening to the melodious songs. In the morning he quietly went away. Thus the young wife saved her husband from the clutches of death. Since then this day of Dhanteras came to be known as the day of "Yamadeepdaan" and lamps are kept burning throughout the night in reverential adoration to Yam, the god of Death.

According to another popular legend, when the gods and demons churned the ocean for Amrit or nectar, Dhanavantri (the physician of the gods and an incarnation of Vishnu) emerged carrying a jar of the elixir on the day of Dhanteras.

There is a peculiar custom in Maharashtra to lightly pound dry coriander seeds with jaggery and offer as Naivedya (Sacred Offering). In villages cattle are adorned and worshiped by farmers as they form the main source of their income. In southern India cows are offered special veneration as they are supposed to be the incarnation of Goddess Lakshmi and therefore they are adorned and worshiped on this day.