From Wikipedia,
Holi is a Hindu Spring annual festival. It takes place over two days around late March or early April as per the Hindu calender. On first day, a bonfire is lit at night to signify the burning of Holika. On the second day, known as Rangapanchami or Dhulandi, people go around throwing colours at each other and meet and have fun. A special drink called thandai or bhang is also consumed sometimes, which actually contains small amounts of marijuana to make the festival more enjoyable. People invite each other to their houses for feasts and celebrations.
As for my own experience of bhang, I have none 
