The History of St. Patrick's Day:
Saint Patrick was born around 385 AD in the United Kingdom. His real name is believed to be Maewyn Succat (he took on Patrick, or Patricus, after becoming a priest). When he was 16, he was kidnapped by a group of Irish raiders and sold into slavery in Ireland. During his six-year captivity as a shepherd, he began to have religious visions and found strength in faith in God. He finally escaped to France and became a priest and then later on became a bishop. When he was around 60 years old, Saint Patrick returned to Ireland to spread the Gospel (the Christian Word). He used the shamrock, which resembles a three-leafed clover, as a metaphor to explain the Christian concept of God (Father, Son and Holy Spirit).
Green is associated with St. Patty's Day because it is the color of spring, Ireland and the shamrock.
Leprechauns are also associated with St. Patty's Day, although the only reason they are is because they're Irish.