Jan A Wagenaar, Origin and Transformation of the Ancient Israelite Festival Calendar, Harrassowitz Verlag, Wiesbaden 2005, proposes that the Feast of Unleavened Bread was originally in the month of Iyar, not Nisan. He argues this by pointing out that the feast was originally at the time of the barley harvest (Exodus 9:31-32) and not assigned to a fixed date in the calendar. He asserts, citing D.C. Hopkins, The Highlands of Canaan: Agricultural Life in the Early Iron Age, SWBAS, Sheffield, 1985, that the climate of Iron Age Palestine is about the same as today’s. Then he notes that the barley harvest in the lowlands of what is now Israel begins at the beginning of May, and later in the highlands. For this he cites a study of Palestinian agriculture from the first half of the 20th century, G. Dolman, Arbeit und Sitte in Palästina, Gutersloh, 1928-1942. He quotes from Dolman 4 dates for the beginning of the barley harvest near Jerusalem: June 3rd, 1911, May 21 1913, May 16th 1921, and May 24th 1925. This is too late to fall consistently in the Babylonian month of Nisanu which in the time of Artaxerxes I (reigned 464 BCE-424 BCE) began as early as March 26th, though as late as April 23rd. (R.A. Parker and W.H. Dubber stein, Babylonian Chronology 626 B.C. - A.D. 75, Brown University Press, 1956).
The hypothesis that the Feast of Unleavened Bread could not have fallen always in Nisan in the late Iron Age depends on the proposition that the climate of modern Israel is the same as in the Iron Age kingdoms of Israel and Judah. Does anyone have access to climatological studies that would refute or confirm this? I found this study on-line, in which one author holds that
the climate of the eastern Mediterranean generally has fluctuated in a fairly narrow range since the latter half of the EB IV.
which would support Wagenaar’s hypothesis.