The main celebration was Tuesday, but the festivities carry on until today.
http://www.diwalifestival.org/diwali-calendar. html
It's predominantly a festival of light (hence the fireworks). The first story I learnt was that it's a celebration of Lord Rama's return from the forest after defeating a demon, but there are many other stories associated with it.
For me, it's when all my Indian colleagues come to work dressed in the most gorgeous clothes and share some of the most scrummy Indian sweets you'll ever taste.