Lewis & Short

septennĭum, ii, v. septuennium.

septŭennĭum, ii, n. [septuennis], a period of seven years, Fest. p. 347 Müll.
Collat. form septennĭum, Prud. ap. Tetrast. 6; Vell. 2, 31, 2 Haase.