For us, Nizari Ismailis, the night of October 4, 2005, was the first night of Ramadan. This holy month for Muslims coincides with Rosh Hoshana.
Also, the Bohras (official website), or Mustalian Ismailis, like the Nizari Ismailis, do not rely on moon sighting, but on astronomical calculation. I was always under the impression that we shared our calculated calendar from the Fatimid period, but since they started a day earlier, there must a divergence somewhen. Anybody know when and/or why?
Update: Official website of the Dawoodi Bohras linked as Aziz suggested in comments. Also, did some research on the calendar (and conferred with a Bohra friend who did her PhD at Harvard and is now at Chicago) and was reminded that the Nizari calendar was further elaborated on by Nasir ad-Din Tusi during the post-Fatimid era. So, while there is a great deal of agreement between the Mustali and Nizari calendars, there are occasional – although not persistent – differences. Of course, what we haven’t figured out is why the Sulaymani Bohras (another Mustali group) always seem to agree with the Nizari calendar.
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