From Wikipedia: http://en.wikipedia.org/wiki/T%E1%BA%BFt
Tết or Vietnamese New Year, is the most important celebration of Vietnamese culture. The word is a shortened form of Tết Nguyên Đán “Feast of the First Morning of the First Day”. The current new year beginning January 31 is the year of the Horse. It celebrates the arrival of spring based on the the lunisolar calendar.
Tet takes place from the first day of the first month of the Vietnamese calendar (around late January or early February) until at least the third day. Many Vietnamese prepare for Tết by cooking special holiday foods and cleaning the house. There are a lot of customs practiced during Tết, such as visiting a person’s house on the first day of the new year (xông nhà), ancestral worshipping, wishing New Year’s greetings, giving lucky money to children and elderly people, and opening a shop.
Tet is also an occasion for pilgrims and family reunions. During Tết, Vietnamese visit their relatives and temples, forgetting about the troubles of the past year and hoping for a better upcoming year. They consider Tết to be the first day of spring, and the festival is often called Hội xuân (spring festival). These celebrations can last from a day up to the entire week, and the New Year is filled with people in the streets trying to make as much noise as possible using firecrackers, drums, bells, gongs, and anything they can think of to ward off evil spirits. This parade will also include different masks, and dancers hidden under the guise of what is known as the Mua Lan or Lion Dancing.
The Lan is an animal between a lion and a dragon, and is the symbol of strength in the Vietnamese culture that is used to scare away evil spirits. After the parade, families and friends will come together to have a feast of traditional Vietnamese dishes, and share the happiness and joy of the New Year with one another. This is also the time where the elders will hand out red envelopes with money to the children for good luck in exchange for Tết greetings.