He made an epitome of the encyclopedic treatise in many volumes De verborum significatu, of M. Verrius Flaccus, a celebrated grammarian who flourished in the reign of Augustus. Festus gives the etymology as well as the meaning of every word; and his work throws considerable light on the language, mythology and antiquities of ancient Rome. He made a few alterations, and inserted some critical remarks of his own. He also omitted such ancient Latin words as had long been obsolete; these he apparently discussed in a separate work now lost, entitled Priscorum verborum cum exemplis. Even incomplete, Festus' lexicon reflects at second hand the enormous intellectual effort that had been made in the Augustan Age to put together information on the traditions of the Roman world, which was already in a state of flux and change.
Of Flaccus's work only a few fragments remain, of Festus's epitome only one damaged, fragmentary manuscript. The rest is further abridged in a summary made at the close of the 8th century CE, by Paul the Deacon.
The sole surviving Festus manuscript, the Codex Festi Famesianus at Naples, is an 11th century manuscript. When it was rediscovered during the early Renaissance half of it was already missing, so that it only contains the alphabetized entries M-V, and that not in a perfect condition. It has been scorched by fire and disassembled.
Collating these fragmentary abridgements, collating them and republishing them with translations, is a project being coordinated at the University of London with several objectives in view: to make this mass of information available to researchers in a usable form; to stimulate debate on Festus and on the Augustan antiquarian tradition on which he drew, and generally to enrich and renew studies on Roman life, on which Festus provides such essential information.
Festus' modern editors sum up his importance: