“Of far greater importance, however, is a much later document from the Cairo Geniza: a letter of a Babylonian exilarch - one of the main leaders of the Rabbanite community - with detailed calendrical instructions for the year 835/6 CE. The letter reveals that Passover (15 Nisan) in that year was due to occur on a Tuesday; whilst according to the present-day rabbinic calendar, it should have occurred on Thursday. According to the exilarch, the setting of Passover on Tuesday was dictated by a concern to avoid visibility of the new moon before the first day of the month. This concern does not exist in the present-day rabbinic calendar. Once discovered and published in 1922, the exilarch's letter proved beyond doubt that almost five hundred years after R.Yose and 'Hillel the Patriarch', then fixed calendar in its present-day form had still not been instituted." (Sacha Stern, Calendar and Community: A History of the Jewish Calendar, p.184-5)
