The absence of month names from the Homeric epos led M. Nilsson (1918) to the conclusion that the Greek calendar, religious in origin and character, was nothinq else than a post-Homeric invention, essentially of the 7th century B.C.. He even considered the only month (Ληναιών) mentioned in Hesiod’s Works and Days as a later interpretation. Sound objections to Nilsson’s argument, as regards the date of appearance of the Greek calendar, were first raised by G. Thomson (1943), who supported its Mycenaean provenance. Thomson was absolutely justified by the deciphering of the Linear B tablets (1952). We know now, on the basis of the categorical account of the palatial archives of Knossos and Pylos, that the Mycenaeans had in fact established local calendars, with differents month names, obviously. However, structured on a common base for counting the time. The first part of this article underlines the striking analogies and convergences between the Mycenaean calendars and those of the historic period, having as an objective the reinforcement of Thomson’s view, on the grounds of new relevant data. A special emphasis is laid on the fact that the Mycenaean calendars, like the later Greek ones, present a distinct religious character, as various months have been named after deities, sanctuaries or festivals (e.g. pa-ki-ja-ni-jo-jo me-no, di-wi-jo-jo me-no), while at the same time they serve as chronological frames for regulating the official feast calendar and every cult activity. We have every reason to presume that the Mycenaeans would have sought a calendar model in the flourishing palatial Crete, from where they had also adopted writing, and the metric and measuring system. The second part of the article deals with our knowledge of the Minoan calendar, for which there is not any direct and explicit written information. Evidence is also examined (iconography, Homeric abstract on the renewal of King Minos” sacred reign, Odyssey, T 176-179), which seems to indicate the use of a luni-solar calendar of eight-years uration. This view is substantially supported by the archaeoastronomical observations, conducted recently by scholars of the Uppsala University in the Petsophas and Traostalos peak sanctuaries on Crete. At least some of the numerous Minoan peak sanctuaries seem to have functioned not only as religious and cult sites, but also as astronomical observatories for regulating the calendar cycle, to which the priesthood contributed significantly, according to the well known Egyptian triptych “astronomy-religion-calendar”.