A search on Google books produced an elderly reference, Wm Ramsay, Ovid: selections for the use of schools (1868) discussing how the Julian year worked (p.333). But as with so many of these old sources, the referencing to ancient texts is really quite good.
In giving an account of the Roman Calendar, it will be convenient first to explain that portion of the subject concerning which our information is full and complete; and then to pass on to the consideration of those points which are comparatively doubtful and obscure. According to this plan, we shall commence at once with an account of the constitution of the Julian Year [1]. …
[1] The principal authorities are Plutarch, Vit. Caes. 59, Dion Cassius 43. 26, Appian. B.C. II, Ovid Fast. 3. 155, Sueton. Jul. 40, Plin. H.N. 18. 25, Censorinus 20, Macrob. S. I. 14, Ammian. Marcell. 26. 1.
Which gives us something to work with. It will be interesting to see what Ramsay says…