One of the more popular legend associated with Holi is the love story between Krishna, one of the major Hindu deity, and Radha, a Hindu goddess. The story goes that Lord Krishna was unhappy with his dark skin tone compared to Radha who had a fair complexion. Playfully suggested by his mother, Yashoda, Lord Krishna then smears color on Radha's face to change her complexion to his desire.
Although the dates differ every year, it is a celebration of the arrival of spring and is on the day after the full moon in March. This year, the Holi Festival is celebrated on March 21. The celebration begins a day before Holi, Holika Dahan, with a large bonfire that is lit to burn out evil spirits, on March 20.
The colorful powder, or gulal in Hindi, used for the Holi Festival were traditionally made from using natural sources, including plants and flowers, such as turmeric. Although nowadays, cornstarch with color dyes, are most often used, so maybe having a pair of goggles may come in handy if you decide to take part in the custom.