Here’s my theory about the ten-night festival of Grannus, celebrated (at least) at Limoges as the decamnoctiactis Granni (AE 1989: 521). That inscription does not say on which days of the year that festival falls, however—or even in which calendar its dates were reckoned. There is, however, an inscription commemorating the dedication of an altar to Apollo Grannus on this date—a. d. XV Kal. Sept., or August 18—from Altiaia (Nesselhauf, BRGK 27: 1937, 88). Now, Altiaia is far away from Limoges. It’s the present-day Alzey in Rheinhessen near Mainz, an area probably belonging to the Treveri before being reassigned to Germania Superior (perhaps in Flavian times). The dedication of this altar certainly proves nothing about the festival in Limoges. However, the dates altars were dedicated were often observed afterwards as feast days in the cult of the god concerned. For somebody like me, who tries to follow the Treveri anyway, this is clearly a date worth observing. There’s one other datum I want to bring to bear in the discussion: Six nights ago (counting inclusively in the Roman fashion) was the Ides of August. One of the most important components of this over-determined holiday was the festival of Diana Nemorensis, the Nemoralia. It should not surprise us, then, that one of the few dated inscriptions mentioning Diana in Gaul are dated to the Ides of August (CIL xiii: 6629). This is from Stockstadt am Main—not far from Alzey, but beyond the Taunus on the Germanic limes. There a military unit dedicated an altar not only to Diana … but also to her twin brother Apollo, showing that he too was in dedicants’ minds at this time.
The sources for this reconstruction are a bit disparate: Limoges, two sites in Germania Superior, the Roman fastí. The geographic range goes even farther considering that the military unit at Stockstadt included Britons. Since most inscriptions to Apollo Grannus have been found in northern Gaul and the Germanies, it shouldn’t surprise us that that’s where these dated inscriptions come from.
If we want to choose a date to observe the decamnoctiacis Granni, why not have them run from the Ides of August with Nemoralia through the 18th, when the Altiaia altar was dedicated? If the 18th were the last night, we would take the decamnoctiacis to begin on the 9th, or a. d. V Id. Aug. (Alternatively, we might take the decamnoctiacis to begin on the Ides and then end on the 22nd, the day before Volkanalia. The 18th would then seem to lose some of its significance—although, for what it’s worth, it would then begin the second half of the ten-night festival.)
This is, in any case, an excellent day to honour Apollo Grannus, as did Martius Senopatius Novellus when he dedicated the altar of Altiaia 1,839 years ago.
Aué Apolló Granne!
Aué, deus saluifice!
Tú morbós dépellis, animósque mortalium illúminás. Gratiá tuá úniuersus uenustus fit, homines húmánitátem discunt.
Té bonás precés precor uti sies uolens propitius mihi, domó familiaeque meae ceterísque cultóribus deórum Galliárum.
Aué, Granne deus! Maximé salué.