Each camp had its own religious personnel; standard bearers, priestly officers and their assistants, including a haruspex, and housekeepers of shrines and images. A senior magistrate-commander (sometimes even a consul) headed it, his chain of subordinates ran it and a ferocious system of training and discipline ensured that every citizen-soldier knew his duty. As in Rome, whatever gods he served in his own time seem to have been his own business; legionary forts and vici included shrines to household gods, personal deities and deities otherwise unknown. From the earliest Imperial era, citizen legionaries and provincial auxiliaries gave cult to the emperor and his familia on Imperial accessions, anniversaries and their renewal of annual vows. They celebrated Rome's official festivals in absentia, and had the official triads appropriate to their function – in the Empire, Jupiter, Victoria and Concordia were typical. By the early Severan era, the military also offered cult to the Imperial divi, the current emperor's numen, genius and domus (or familia), and special cult to the Empress as "mother of the camp." The near ubiquitous legionary shrines to Mithras of the later Imperial era were not part of official cult until Mithras was absorbed into Solar and Stoic Monism as a focus of military concordia and Imperial loyalty.
Even in other places, of what did the legions keep observance?
Rome's official festivals