Despite their elusiveness, the people referred to as “Celts” by ancient chroniclers left behind certain archaeological remains that may be interepreted from the perspective of archaeoastronomy in an attempt to discover a calendrical “root” for them. In recent years, a number of studies on Late Iron Age sites, Roman or romanised locations and Christian landscapes in Hispania and in Gaul raised the possibility of detecting physical evidence of the celestial concepts that some classical authors attributed to the Celtic mystics, the Druids. However, these studies dealt with certain key aspects of how the Celts organised time that are not generally known and which tend to be presented in a summary way. Here, we explore aspects such as the difficulty of referring to a "Celtic calendar" per se, the sources for our study, the difficulties of adjusting the cycles of the Sun and Moon, the role of the “horizon calendars” and how these aspects may have played a role in actions that left a physical footprint that can still be seen today at several archaeological sites. We show that, although there may be common aspects that connect all Celtic sites and areas, there was no common calendar as such, although there are solid indications of the usage of a shared time-reckoning system.