Holi is a joyous celebration of colour and mirth. It celebrates the end of Winter, the arrival of Spring, the end of the darkness of winter and the light of spring.
It is celebrated with dancing, tomfoolery, music and splashes of colour. This is a day that people can get away with mischievous acts of dousing passers by with coloured powders, water cannons (pitchkaris), pranks and general playful mischief. Any if anyone gets upset, the rousing retort is, “Don’t mind, it’s Holi!” (Hindi = Bura na mano, Holi hai.)
Here are sites with more information:
http://www.bbc.co.uk/religion/religions/hinduism/holydays/holi_1.shtml