Diwali (also Dipavali), the festival of lights, is an important Hindu festival lasting several days.
The festival celebrates the victory of good over evil, truth over lie, light over shadow and life over death.
Originally, oil lamps were placed in the windows and doors, but today the houses are often decorated with numerous fairy lights. In addition, firecrackers and other fireworks are lit everywhere in the streets. 
For a European, the festival of lights seems a bit like a mixture of Christmas and New Year.  ;-)