The Collaborative International Dictionary of English v.0.48:

Septennial \Sep*ten"ni*al\, a. [L. septennium a period of seven
   years; septem seven + annus year. See Seven, and Annual.]
   1. Lasting or continuing seven years; as, septennial
      parliaments.
      [1913 Webster]

   2. Happening or returning once in every seven years; as,
      septennial elections in England.
      [1913 Webster]