The Collaborative International Dictionary of English v.0.48:

Black Monday \Black" Mon`day\
   1. Easter Monday, so called from the severity of that day in
      1360, which was so unusual that many of Edward III.'s
      soldiers, then before Paris, died from the cold. --Stow.
      [1913 Webster]

            Then it was not for nothing that my nose fell a
            bleeding on Black Monday last.        --Shak.
      [1913 Webster]

   2. The first Monday after the holidays; -- so called by
      English schoolboys. --Halliwell.
      [1913 Webster]