Festus, Epitome of Flaccus’ ‘On the meaning of words’ 136.6-10(L)
The rather sad tale of the works of Flaccus and Festus is told here. This is one of the surviving remnants:
Martius mensis initium anni fuit et in Latio, et post Romam conditam, quod ea gens erat bellicosissima; cuius rei testimonium est, quod posteriores menses, qui annum finiunt, a numero appellati, ultimum habent Decembrem.
The month of March [named after the god Mars] was the beginning of the year both in Latium and after the foundation of Rome, because that people was very warlike. The evidence for this fact is that the later months, which round off the year, are named after their number, and have December as the last one. [i.e., December, the last month of the year, takes its name from decem = 10, so refers to itself as ‘the tenth month’.]