The subject to be considered is the festival of Hermes Kriophoros in Tanagra. A brief Pausanias’ reference (9.22.1) contains very few but remarkable details of the celebration as it was held in Late Antiquity. All indications concerning the procedure and the content of the festival inferred from Pausanias’s description are analyzed in combination with the attested characteristics and attributes of Hermes, as well as with the religious symbolism universally attached to the ram, the divine shepherd, and ritual circular movement. The ethnographic evidence for similar rituals is also adduced. It is concluded that the Tanagran festival originated in an ancestral communal ceremony of annual territorial lustration. That ceremony was linked to the increase of solar activity in midspring and therefore also included the parallel magical stimulation of the sun’s course. Gradually, that primitive magic ritual would have been elaborated in a more complex seasonal ceremony of stimulation of the generative solar power of the Great Mother-Goddess with the help of a young male ram-god and was eventually transformed into a celebration of the sacred marriage between the Great Mother-Goddess and a young beautiful divine shepherd. In the process of the formation of polis, the festival became a more social celebration, which served to secure the city-state’s secure existence under the protection of Hermes Kriophoros.