Just to see how things lined up, I compiled this list of all the festivals both Roman and "Celtic" that I could find. I got the Roman ones from Wikipedia so I think time would have to be spent on double checking the reliability of the list; it could be missing some or things could be a little out of place/order. I also compiled a list of deities or 'spirits' that are associated with each month through these events. Some of them are tentative sort of guesses in which case I put a (?) to indicate that this is not necessarily established.
January – Agonalia (for Janus), Carmentalia, Paganalia
February – Imbolc, Parentalia, Lupercalia, Quirinalia,
March – Matronalia, Feriae Marti, “sacred fire for Vesta is renewed”, Equirria, Bacchanalia, Agonalia (for Mars), Quinquatria, Festival of Salus
April – Floralia, Veneralia, Megalesia, Ludi Cereales, Fordicia, Parilia, Robigalia
May – Beltaine, Festival for Bona Dea, Lemuria, Mercuralia, Agonalia (for Vediovis),
June – Festival for Bellona, Vestalia, Quinquatrus minusculae, Festival for Summanus,
July – Poplifugia, Ludi Apollinares, Nonae Caprotinae Juno, Sacerdotes Publici, Caprotinia, Neptunalia
August – Lughnasadh, Opalia, Vertumnalia, Nemoralia, Portunalia, Vinalia Rustica, Consualia, Vulcanalia, Opiconsivia, Volturnalia
September – Ludi Roman
October – Samhain, Fast of Ceres, Meditrinalia, Festival for Fontus, Equirria, Armilustrium
November – Pomonia, Epulum Jovis, Festival for Feronia, Brumalia (starts)
December – Saturnalia, Rites for Bona Dea (women only), Faunalia, Agonalia (for Sol Indiges), Consualia, Eponalia, Opalia, Divalia, Larentalia, Dies Natalis Invicti Solis
January – Janus, Carmenta
February – Brighid, Pan/Faunus/Lupercus , Ancestors (helpful dead), Quirinalia
March – Juno, Mars, Vesta, Bacchus, Minerva, Salus
April – Flora (goes in to May), Venus, Cybele, Ceres, Terra, Pales, Rogibus
May – Belenus(?), Aengus(?), Lemures (harmful dead), Bona Dea, Mercury, Veiovis
June – Bellona, Vesta, Minerva, Summanus
July – Jupiter, Apollo, Juno, Neptune, Consus
August – Lugh, Donn, Tailtu, Ops, Vertumnus, Diana, Portunes, Venus, Consus, Vulcan, Volturnus
September – Jupiter
October – Ancestors (both kinds), Aengus(?), Dagda(?), Boann(?), Mars, Ceres, Meditrina, Fontus,
November – Pomona, Jupiter, Feronia, Dionysus/Bacchus
December – Sol Invictus, Sol Indiges, Larenta, Ops, Faunus, Bacchus, Bona Dea, Epona, Saturn, Angerona, Consus
So this is a fairly large list.
ursus_of_unrv mentioned in reply to comments in his earlier post that some of these Roman festivals would have been optional, in that you would choose to honour an event specifically, if it happened to be a deity to which you were particularly attached. I’ve done my best to cut out generic non-deity/religion specific holidays, because we have our own modern civic holidays. It looks like every month is particularly rich with choices except for September. Perhaps it would be a good idea to go through them and try to parse out what they were intended for one by one (this could be cross posted to the religioromana list too).
The Ford article suggests moving some Roman festivals to coincide with Celtic festivals (or vice versa). I don’t really do that personally, because I don’t see a strong need for it. I find that, at least the holidays I’ve celebrated, line up broadly in theme in some smaller ways at least (i.e. Imbolc in February in honour of Brighid, and the ritual relighting of the Vestal flame in March both have fire/hearth focus) with the holidays around them. Ford moves Parentalia and Lemuria to Samhain, but I find that the Lemuria actually fits better in around Beltane; so I suppose one could move the Parentalia to Samhaim if you wanted some kind of counterpoint to Lemuria being held in around Beltane…
From the huge list above, these are the ones I have been part of or led in the past (some on my own, others as part of an open or closed gathering): Celtic – Samhain, Imbolc, Beltane, Lughnasadh; Roman – Saturnalia (and the festivals in and around it), Parentalia, Lupercalia, Matronalia, Floralia, and Lemuria. I also honour the Lares and Penates, Genius and Juno of our household three times a month on the Kalends, Nones, and Ides. This includes offerings of incense, wine, and food (usually spelt cookies, which contain ginger, honey, milk, and spelt flower – all organic). I honour Brighid and either Janus, Juno, or Jupiter respectively on these occasions; the Kalends for Janus, the Nones for Juno, and the Ides for Jupiter. Obviously I'm still working things out here; it is hard to be sure what festivals and ceremonies are most 'important' to celebrate and what exactly to include in personal rites. What kinds of festivals/ceremonies have you participated in or led? If you honour familial deities, how do you choose to do it? I’m curious because what I have chosen to do is relatively ‘new’, but based on my understanding of how things probably would have been done in the Ancient world. I don’t do a ritual like that daily, but I think there was precedent in Antiquity for doing such a thing on the Kalends, Nones, and Ides at any rate.