happy thanksgiving to you , too <| sep |> Probably the original reason was that Jesus sat at the last supper with 12 disciples , and one of them was a betrayer . When Judas Iscariot the betrayer left early to go conspire with the Pharisees there were then 12 . So 13 became a number associated with evil . Thus Copypasta from The Encyclopedia of the Unusual and UnexplainedIt has long been a matter of etiquette in France to avoid having exactly 13 guests at a dinner or party . Napoleon 17691821 wouldn ' t allow a dinner to begin if there were 13 guests at the table . There is a custom of the quartrozieme , a professional guest who can be called on short notice to avoid having only 13 people dining at a dinner party . Although the superstition of 13 guests is not quite so strong in the United States , President Herbert Hoover 18741964 would not permit a gathering of 13 while he was in the White House . President Franklin D . Roosevelt 18821945 had the same superstition , and it is said that his personal secretary was often called upon to be the 14 th guest at a dinner party .