It is best known for riotous scenes of revellers throwing water or coloured powder over one another. This is done in celebration of the end of winter and as a reminder of the cooling monsoon rains to come (usually in July or August).
Holi is a Hindu festival, celebrated at Phalguna Purnima (Full Moon) at the end of winter's last lunar month. It lasts 24 hours: from sunset on the day before the festival (when Holika bonfires are lit around the country, and people congregate to dance and sing) until sunset of the day of the festival.
Be sure to wear old clothes if you are going out: you will not be asked before you are pelted.
Whilst there are usually a few complaints about some locals being overly enthusiastic, the overwhelming majority of people get into the spirit of the festival. Great Britain's Prince Harry was one such reveller in his March 2016 trip to Nepal