International Sweater Vestival or Vestival is a wear a sweater vest to work holiday that is a parody of other celebrations. It takes place on the second Friday after Thanksgiving. In 2014, International Sweater Vestival will fall on Friday, December 5th.
Knoxville, Tennessee's unrelated version of Vestival is in its eighth year as a festival and occurs in May, rendering sweater vest wearing impractical unless it is cool out.
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All details taken directly from provider content at http://en.wikipedia.org/wiki/International_Sweater_Vestival