On the Gregorian calendar (so named after Pope Gregory XIII) this September, two of the holiest Jewish and Islamic high holidays coincide. On September 10, Jews around the world began their high holidays by commemorating Rosh Hashana which continues for 10 days until September 19, Yom Kippur. It is on that day that Muslims in general, and Shia in particular, commemorate the sacred day of Tasu’a and then a day later – Ashoura – the 9th and 10th of the Islamic month of Muharram respectively.
Both the Jewish and Islamic calendars are lunar while the Gregorian calendar on which they are being cast is solar. The lunar calendar is 11 days shorter than the solar and for that reason, though stable on their own respective calendars, Jewish and Islamic holidays appear to “roam” aimlessly on the Christian calendar.
None of this is “the fault” of either the Jewish or the Islamic calendar. Given the way in which the Christian calendar has been imperially universalised, the other two may look erratic and confusing, but Jewish and Islamic high holy days are perfectly logical, routine, and regular.