There is a long-lasting debate, started in the nineteenth century by d‟Arbois de Jubainville and Jullian, among others, regarding the pan-Celtic nature of the mid-season (or rather, start of season) feasts known from the Mediaeval Ireland (set on 1st November, February, May and August). D‟Arbois indicated that one of these feasts coincided with the festival celebrated during Roman times at Lugdunum (Lyon) on August 1st from 12 BC onwards. We recently verified that the orientation of the earlier parts of this Roman colony were laid out facing the sunrise on this date since its foundation in 43 BC, prior to any possible link with Augustus. This fact prompted us to investigate the orientation of other Roman cities in Gaul, particularly those named Lugdunum which still contain Roman buildings capable of being measured. The most complex of these is Lugdunum Conuerarum (present-day Saint-Bertrand-de-Comminges). With a Celtic name and mixed Aquitaine-Latin culture, it is oriented towards the sunrise on February 1st. Other cities in Gaul and Hispania have also been considered, which have similar orientations. We therefore verify the pan-Celtic character of the mid-season feasts. Finally, we present the hypothesis that the conversion of these feasts from a luni-solar calendar to the solar Julian calendar took place in the centre of Gaul at some time between the reigns of Caesar and Augustus. At a later stage, this model would be exported by early Christianity into Ireland, then serving as an interpretative inspiration for scholars such as d‟Arbois de Jubainville and others.