Lewis & Short

septĭmum, adv., v. septimus, I. 2.

septĭmus (septŭmus), a, um, num. ord. adj. [septem].

  1. I. In gen., the seventh: mense septumo, Plaut. Am. 1, 2, 20: septimas esse aedes a portā, id. Ps. 2, 2, 3; cf.: isque Septimus a prisci numeratur origine Beli, Ov. M. 4, 213: Roma condita est secundo anno Olympiadis septimae, Cic. Rep. 2, 10, 18: septimo die, id. Tusc. 3, 26, 63: sententia septima decima, id. Clu. 27, 74; also n one word, septimodecimo die, Vitr. 9, 1 (4) fin. et saep.
    Hence, advv.
      1. 1. Die septimi, on the seventh day (like die crastini, noni, quinti, etc.): ibi mercatum dixit esse die septimi (septimei, Ritschl), Plaut. Pers. 2, 3, 8; cf. Gell. 10, 24, 1 sqq.; Macr. S. 1, 4.
      2. * 2. septĭmum, for the seventh time: Marius tam feliciter septimum consul, Cic. N. D. 3, 32, 81.
      3. 3. septĭmō.
        1. * a. For the seventh time: C. Marium creatum septimo consulem, Quadrig. ap. Gell. 10, 1, 3.
        2. b. Seven times (post-class.): lavit ad diem septimo aestate vel sexto: hieme secundo vel tertio, Treb. Gall. 17; Vopisc. Flor. 4.
  2. II. In partic.: septimus casus, the instrumental, adverbial, etc. case (distinguished from the casus sextus, with prepositions), Quint. 1, 4, 26; Diom. p. 302 P.; Prisc. p. 673 ib.; Cledon. p. 1863 ib.