Modern reconstructionists often assume – not implausibly, at first blush – that Romans observed Roman festivals, and Greeks, Greek festivals. If one compares the city of Athens in the classical period with Rome in the mid Republic and nothing else, this may hold some water. But in the Roman imperial period, from which the bulk of surviving literature comes, Roman festivals were common across the Greek-speaking world, and what festivals were called depended on the language of the speaker, not the origin of the holiday. The Saturnalia, for example, were so popular in the East that the Greco-Assyrian satirist Lucian wrote more than one work about them, yet he always called them Kronia, and took for granted that the god worshipped at this festival was the same as the father of Zeus mentioned by Homer and Hesiod (and so, to be clear, did Roman authors!).
Here, we have gathered some lists of festivals (gr. heortai, lat. dies festus). from bilingual texts, to illustrate this point. Firstly, from the Hermeneumata Leidensia (with the Roman festival dates added): –
Kronia = Saturnalia [December 17-23]
Dionysia = Liberalia [March 17]
Panathenaea = Quinquatria [June 13-15]
Poseidonia = Neptunalia [July 23]
Hephaestia = Vulcanalia [August 23]
Horothesia = Terminalia [February 23]
Nekysia = Parentalia [February 13-21]
Hestiaea = Vestalia [June 7-15]
Ennead (enneas) = Nundinae (‘eight-day week’)
Genethlia = Natalicia (‘birth day’)
From the Hermeneumata Monacensia (omitting doublets):
Neomenia = Novendiale [!]
Trope khimerine (‘winter solstice’) = Bruma (i.e., Brumalia) [November 24-October 24]
Neon etos = Novus annus (‘new year’)
Lykeia = Lupercalia [February 15]
Thesmophoria = Cerealia [mid- to late April]
Apollonaria [!] = Apollinaria [July 5-13]
Serapeia = Serapia [April 25]
Isia = Isia [multiple holidays]
Megalesia = Dindyma (i.e., Megalensia) [April 4-10]
Aphrodisia = Veneralia [April 1]
Time syngeneon (‘honoring of relatives’) = Cara cognatio [February 22]
With some of these festivals, translatability was entirely unproblematic, because whatever their specificities in observation, they were conceptually straightforward. The Cerealia remain understood in roughly the same way, whether they are translated as Thesmophoria or more blandly as Demetria, as in the grammarian Charisius. No one could misunderstand the translations Neptunalia=Poseidonia, Saturnalia=Kronia, Vulcanalia=Hephaistia, and Dionysia can be translated as Bacchanalia or Liberalia with little difference in meaning.
On the other hand, other festivals may have a translation, but this in itself does not make them understandable, because they are more culturally specific. We can cite two instances from Charisius (who wrote a grammar of Latin for native Greek-speakers):
Parentalia: Nekysia. At which they worship those who died abroad, [which(?)] is called parentarium.
Terminalia: the checking and fixing of the boundaries, to whom the Romans sacrifice at this festival.