Lewis & Short

Sextīlis, e, adj. [sextus], sixth, only with mensis, of the month of August: MENSE SEXTILI, S. C. ap. Macr. S. 1, 12 fin.: Sextili menso caminus, Hor. Ep. 1, 11, 19.
Hence, subst.: Sextīlis, is, m. (sc. mensis), the sixth (month); hence, the month of August, acc. to the old Roman reckoning (counting from March), afterwards called Augustus (v. h. v., and cf. Varr. L. L. 6, 4 fin.), Cic. Fam. 10, 26, 1: Sextilem totum mendax desideror, Hor. Ep. 1, 7, 2: Kalendae, of August, Liv. 3, 6; 6, 1 fin.: Nonae, Idus, id. 41, 16.