From the Holi Wikipedia page:
Holi is a Hindu spring festival in India and Nepal, also known as the festival of colours or the festival of sharing love. Holi is a two-day festival which starts on the Purnima (Full Moon day) falling in the Bikram Sambat Hindu Calendar month of Falgun, which falls somewhere between the end of February and the middle of March in the Gregorian calendar. The first day is known as Holika Dahan or Chhoti Holi and the second as Rangwali Holi, Dhuleti, Dhulandi or Dhulivandan.
It is an ancient Hindu religious festival which has become popular with non-Hindus in many parts of South Asia, as well as people of other communities outside Asia.
It is primarily observed in India, Nepal, and other regions of the world with significant populations of Hindus or people of Indian origin and Nepalese diaspora. In recent years the festival has spread to parts of Europe and North America as a spring celebration of love, frolic, and colours.
Holi celebrations start on the night before Holi with a Holika bonfire where people gather, do religious rituals in front of the bonfire, and pray that their internal evil should be destroyed as the bonfire starts. The next morning is celebrated as Rangwali Holi - a free-for-all carnival of colours, where participants play, chase and colour each other with dry powder and coloured water, with some carrying water guns and coloured water-filled balloons for their water fight. Anyone and everyone is fair game, friend or stranger, rich or poor, man or woman, children and elders. The frolic and fight with colours occurs in the open streets, open parks, outside temples and buildings. Groups carry drums and other musical instruments, go from place to place, sing and dance. People visit family, friends and foes to throw coloured powders on each other, laugh and gossip, then share Holi delicacies, food and drinks.
Holi is celebrated at the approach of the vernal equinox, on the Phalguna Purnima (Full Moon). The festival date, which is determined by the Hindu calendar, varies from year to year on the Gregorian calendar, typically coming in March, sometimes in February. The festival signifies the victory of good over evil, the arrival of spring, end of winter, and for many a festive day to meet others, play and laugh, forget and forgive, and repair broken relationships, and is also celebrated as a thanksgiving for a good harvest.