Problems with the Hebrew Calendar.
There was a controversy in the Talmud about whether the world was created in Tishri or Nisan; it was decided that the answer is Tishri.
There are also a number of adjustments that have be made continually to ensure that the first day of Rosh Hashana only occurs on a
Monday, Tuesday, Thursday or Saturday. This is to ensure that Yom Kippur, a day on which no work can be done, can never fall on Friday
(the day prior to the Sabbath), or on Sunday (the day after Shabbat) to avoid having the previous day's restrictions still going on
at the start of Sabbath, or the restrictions of Shabbat still going on at the start of Yom Kippur. Hoshana Rabbah, the Day of the Final
Sealing of Judgement, occurring on day 21, Tishri, must not fall on a Shabbat because certain ceremonies would be lost for a year.