Tet Nguyen Dan, or Tet for short, is considered the biggest and most popular festival of the year in Vietnam. Celebrated on the first day of the first month in Lunar Calendar, Tet’s celebration is the longest holiday which may last up to seven days (with the exception of Tet 2012 when the holiday is expected to last for 9 days!). Vietnamese New Year in 2013 will last from February 10-13, and in 2014 from January 31st to Feb 4th.

Full detail: http://www.vietnamonline.com/tet.html