Zoroastrianism - The Old Iranian Calendars, Part 2
All the above-mentioned hypotheses about the Y.A. calendar have been based on the supposition that the Persian year, even in Sasanian times, was a vague year of exactly 365 days, without any intercalation whatever in the civil year for making good the difference (of about a quarter of a day) between such a year and the tropic year. This presumption is, however, contrary to our oldest reports of the Iranian calendar by early Muslim astronomers. These reports are expressly to the effect that an intercalation of one month in the Persian year every 120 (or 116) years was more or less regularly carried out in pre-Islamic times.23 It was apparently the idea of coordinating this tradition with the presupposed adoption of the Egyptian calendar system in Iran in the fifth century that led Cavaignac 24 to advance a totally different theory on this matter.
This is based on accepting literally the statements of the Muhammadan astronomers regarding the actual intercalation in the Sasanian period even in the Persian civil year, and at the same time admitting the introduction in Persia of the Egyptian calendar without any change whatsoever (except, of course, for the substitution of Persian names for the months). The prevalent opinion, as it is well-known, is that there were two sorts of year in use: the civil year which was in general use, and the ecclesiastic, used only for religious purposes, that the first was a vague year, and that the intercalation was limited to the religious year. It is also generally believed that, in adopting the Egyptian vague year, the Iranians changed the year's beginning from the season corresponding at that time to the Egyptian New Year (December) to the vernal equinox. Now Cavaignac, though he admits that the Egyptian calendar was introduced in the fifth century BC, is of opinion that originally the Persian month Frawardin, and not the month Dai, stood for the first Egyptian month, namely Toth.
Moreover, according to his theory, though this vague year (without any intercalation) possibly has been since used to a certain extent by the mass of people, nevertheless the Babylonian (or the Old-Persian used in the Behistun inscription) remained the official calendar of Persia until the fall of the Achaemenian empire, after which it was superseded by the Syro-Macedonian calendar, which lasted from Alexander's conquest till the rise of the Sasanian dynasty. He thinks, therefore, that it was in the Sasanian period that the Y.A. or Mazdayasnian calendar became the official and general means of time-reckoning in Persia, and that it was in that epoch that the intercalation in the Y.A. year was instituted, after which the year remained nearly fixed during the Sasanian period with the New Year about the time of the summer solstice. He believes also that the intercalary month was inserted at the first intercalation after Shahrewar, the sixth month (possibly in the fourth century AD) as a second Shahrewar, and that on the next occasion a second Mihr was added to the year and so forth. As a matter of fact, the beginning of the Egyptian year in AD 632 was only ninety days prior to the Persian New Year's Day, the Egyptian being on the 18th March and the Persian on the 16th June, which  difference might be easily interpreted as the consequence of three intercalations of one month each, during the Sasanian period (406 years.)
Of all the different theories proposed about the date of the introduction of the Egyptian calendar system in Persia, i.e. the creation or the official adoption of the Y.A. calendar, only two are, I think, more or less consistent with many of the known facts and supported, to a certain extent, by tangible arguments. These are those suggested by Marquart and Cavaignac. But each of these two theories has, nevertheless, its weak points and is far from being satisfactorily established or indisputable. They cannot, therefore, be considered as a final solution of this difficult problem.
Cavaignac's thesis agrees, it is true, in every respect with Biruni's statements 25 regarding the old Iranian calendar, namely that the pre-Islamic year of Persia was a stable or fixed year beginning at (or near) the summer solstice and maintained around that point by a 120-yearly intercalation of one month. But besides being incompatible with the contents of the Pahlavi books on this matter and with other evidence in favor of the vague year,26 this theory cannot be brought into harmony with what we know of the parallelism of the Persian year with the Armenian, the Cappadocian, the Sogdian and the Khwarazmian years without the assumption of a very unlikely, if not impossible, condition, namely the general application of exactly the same intercalatory system to all the calendars of these different and often politically separate nations. Moreover, it must be pointed out that Biruni himself, who is our principal authority on this subject, is not consistent in this particular point, and his books contain many contradictory passages implying different times for the beginning of the old Iranian year. For instance, his statement regarding the last intercalation, namely that it was the eighth one and that it was executed through the intercalation of a second Aban, i.e. the eighth month (or a second Aban and a second Mihr together), can only be based on the supposition of the original Nawruz (1st day of the month Frawardin) having been on or about the vernal equinox, and of the latter having been always considered theoretically a New Year's Day.
On the other hand, the theory of West and Marquart of placing the official introduction of the Y.A. calendar in the Persian empire in the middle or the last part of the reign of Darius I, and attributing this reform to that monarch himself, who according to these scholars established the first day of the year on the vernal equinox, is also irreconcilable with the contents of the Afrin gahambar and the Bundahishn on this question. According to the first of these two Mazdayasnian literary documents the season festival maidyoshahem corresponds to 15 Tir. But the Bundahishn states expressly that from maidyoshahem till maidyarem the night increases, and from maidyarem to maidyoshahem the night decreases and the day increases,27 though this book interprets maidyoshahem to be the 11th day of Tir (i.e. the first of the five days of that gahambar) probably following its source not very strictly.28 Marquart is certainly right when he expresses  the opinion that the Mazdayasnian traditions are in this respect contradictory and that the different passages of the Bundahishn are not consistent. For while the summer solstice or the time when the night begins to increase in length is put, as we have seen in the above-mentioned passage, on the 11th day of Tir (or, rather, strictly on the 15th), it is declared in another passage of the same book immediately following the former that "in the feast of hamaspathmaidyem that is the epagomenae at the end of the month Spandarmad the days and nights are equal [in length]". Nevertheless, his conclusion does not seem to be incontestable. He apparently considers the last-mentioned passage of the Bundahishn (relating to the equality in the length of the day and night during the five supplementary days of the year), as well as that part of the former passage implying the identity of maidyoshahem with the summer solstice, as authentic; but he thinks that the gloss placing this gahambar about the middle of Tir, and maidyarem about the middle of the month of Dai, is a wrong interpretation added by the author of the Bundahishn to the original tradition, which was based on the lost parts of the Avesta. Therefore he seems to be of the opinion that maidyoshahem was originally, i.e. at the time of the adoption of the Y.A. calendar, on or about the 1st day of Tir, and maidyarem on or about the beginning of the month of Dai.
Although the original concordance between maidyoshahem and the beginning of the month of Tir in the Old-Avestan calendar (i.e. the calendar of the Avestan people before the adoption of the Egyptian system) is more than possible, the traditional and rather canonical fixing of the places of gahambars in the Mazdayasnian months is, nevertheless, certainly based on the older and authentic sources. These  places are given in the part of the Avesta called Afrin Gahambar. Though it is generally believed that those explanatory passages relating to the places of these season festivals are addenda of later date, interpolated as glosses in the original Avestan text, there is no reason to doubt the antiquity of their contents, which I suppose is as old as the introduction or the official establishment of the Y.A. calendar in Iran.29 The gahambars are thus fixed at an early date in these places and are stabilized in the months of the religious and fixed (vihêjakîk) year.
Suggestions for Further Reading
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- Important Beliefs of Zoroastrianism
- The Old Iranian Calendars, Part 1
- Zoroastrianism - Main Concepts
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26. Such as the changing positions of gahambars, the distribution of the months among the four seasons in Bundahishn beginning with the spring, maidyoshahem being the season of cutting the grass according to Visperad, and its place in the middle of the month Tir according to Afrin gahambar, and the two apparently different but really identical dates for Zoroaster's death in Zadspram, as well as the correspondence apparently given to the month Vohuman and Shahrewar in the Pahlavi commentary of Vendidad (i, 4), and some other data discussed by the present writer in BSOS. ix, 1.
28. The real gahambar day in each of the season festivals of five days' duration is most probably the last or the fifth day. But apparently the author of the Bundahishn, notwithstanding the fact that the point of time after which the day decreases and the night increases can only be one day, has considered all the five days of maidyoshahem roughly as the longest days of the year and equal in length. He has perhaps believed these days to be a stationary period, just as he considers the day and night equal in length in all the last five days of the year (in the same chapter). The whole passage relating to the two festivals of solstices, must be a faithful quotation from a very much older source (possibly the lost parts of the Avesta) without any interpolation except for the identification of maidyoshahem with 11th Tir.
29. This part of Afrin gahambar 3, 9-12, dealing with the length of the six seasons and the places of the festivals in the months is, according to Hertel (Die awestischen Jahrenszeitenfeste, Afrinagan, 3, p. 22), found only in seven out of thirty-one manuscripts of Avesta. Nevertheless, Hertel thinks this is taken from the Hadokht Nask of the Avesta.