Diwali or Deepavali is a "Festival of lights"(deep = light and avali = a row i.e., Row of Lights). This festival comes exactly after 20 days from dussehra ends. And also in between mid-October and mid-November.
Apart from India, it is official holiday for country Like Nepal, Sri Lanka, Myanmar, Mauritius, Guyana, Trinidad & Tobago, Malaysia, Singapore and Fiji.
In Hindus, Deepavali is celebrated due to reasons like Marriage of Laxmi with Lord Vishnu and Return of Lord Rama along with Sita Mata and Lakshman from his fourteen year long exile and vanquishing of the demon Ravana, celebrated as Ramnavami/Dussehra. In Jainism, Diwali marks the attainment of moksha or nivana by mahavira in 527 BC.
One More reason to celebrate diwali is that, In Rural India it marked the End of harvest season, that means money from sale of produce which makes villagers to afford/buy new cloths and other things needed for the home.
Basically, its five days festival:
First Day of which called "Dhanteras", Dhan means Wealth and Teras means 13th day as per Hindu Calender. It is celebrated on the thirteenth lunar day of krishna paksha in Hindu calendar month of Ashwin.
On Dhanteras, Hindus consider it very fortunate to purchase gold or silver or at least one or more utensils as it believed that it is sign of Good Luck.
It is also Known as 'Yamadeepdaan' and the house ladies light eastern lamps or deep which kept burning throughout the night glorifing Yama, The GOD of Death.