A religious observance practiced in India, Bangladesh and Nepal practiced on the last full moon day of the lunar month of "Phalguni "in the Hindu calendar, which occurs around the beginning of Spring in March. It is also practiced by the Indian Hindu/Sikh/Jain diaspora in Singapore, Malaysia, Sri Lanka, Burma, Maldives, Mauritus and in western countries like the US and Canada.
It is known as the festival of colors, since Indians celebrate the day by smearing colored powder on each other as a gesture of brotherhood and goodwill.
Christian extremists (like this guy "John Francois" with his misspellings and misgivings about Holi) from the Northeastern parts of India hate this festival because they are illiterate nuts who have been radicalized by racist missionaries from abroad.
"Holi hai" (Holi exists in Hindi) is a common phrase used by North Indians to signify the onset of Holi.