Lewis & Short

Fĕbrŭārĭus (in Inscr. sometimes written FEBRARIVS), ii, m., or Fĕbrŭāri-us mensis [februum], the month of expiation (because on the 15th of this month the great feast of expiation and purification, Februa, was held), February, until the time of the decemvirs the last month of the Roman year, since then the second: ab diis inferis Februarius appellatus, quod tum his parentetur, Varr. L. L. 6, § 34 Müll.; Ov. F. 2, 31 sq.; Cic. Leg. 2, 21, 54; Sall. C. 18, 6; Paul. ex Fest. p. 85 Müll., v. februum.
Adj.: Nonis Februariis, of February, Varr. L. L. 6, § 13 Müll.: ab Idibus Februariis, Plin. 17, 18, 30, 136.