This study is devoted to the calendar of Megara and her colonies, and to the institutional connections between metropolis and apoikiai (« colonies ») in Antiquity. Most of the scholars agree that the Megarian calendar was reproduced, with minor changes, in the colonies during the Archaic period. Nevertheless, it was recently argued that the similar Megarian institutions attested in the colonial cities might reflect the desire of the apoikoi (« colonists ») to create artificially, in latters periods, a Megarian-Dorian identity. This new thesis is subject of debate. Since the transfer of the metropolis’ calendar to the colonies, as supposed for the Megarian colonisation, is paralleled by similar institutional connections attested in the Euboian and Milesian colonial enterprises. Besides, the calendar cannot be dissociated from the religious sphere, and several months were consecrated to the celebration of particular Megarian cults. This has not stopped the evolutions and the innovations: some months (Bosporios, Herakleios, Potamios) can be connected with new cults and festivals appeared in latter times in the colonies, or with the presence of Greeks of non-Megarian origin in the apoikiai, especially the Boeotians. Megara and her colonies certainly not possessed identical calendars in Archaic period, but the differences were probably less important than the similarities.