Servius, Commentary on Aeneid 1.23:
SATURNIA is an example of antonomasia, not an epithet: this happens whenever that which can be conjoined with a proper name and called an epithet is used in place of the proper noun. Vergil used ‘Saturnia,’ however, as a name befitting Juno’s cruelty, for whenever he joins the name of Saturn either to Jupiter or Juno, he connects to them the causes of their cruelty, as in the line ‘Nor does Saturnius look upon these things with just eyes,’ and elsewhere, ‘you are the sister of Jupiter and the other offspring of Saturn: you turn about such great waves of rage beneath your breast.’
SATURNIA antonomasia est, non epitheton; quae fit quotiens pro proprio nomine ponitur quod potest esse cum proprio nomine et epitheton dici. ‘Saturnia’ autem nomen quasi ad crudelitatem aptum posuit; Vergilius enim ubicumque Iovi vel Iunoni Saturni nomen adiungit, causas eis crudelitatis adnectit, ut “nec Saturnius haec oculis pater aspicit aequis” . et alibi “es germana Iovis Saturnique altera proles: irarum tantos volvis sub pectore fluctus” .