This chapter provides the indications to form a clear idea of the essential features of the prehistoric Iranian calendar. Hundreds of iconographies found on seals and vases from all over Iran and Mesopotamia demonstrate that such a continuity actually existed. The chapter mentions some typical features observable time and again that indicate terrestrialism and interpretation in characterizing the historical period. In late Babylonian times the first month of the lunisolar year, Nisannu, was the one whose neomenia occurred about the time of spring equinox. In the Later Avestan calendar, four of the month names, called after Old Iranian deities including Tistrya-Sirius, were taken over. The coincidences between month and day names were duly celebrated by festivals. Among them, two seem of special interest because they may reflect, as pointed out by Taqizadeh, the astronomical situation at the time of the introduction of the Magian day-names, for example the alleged 'reform' of 441 BC.