Topic summary
Hebrew month
Extracted from the Wikipedia article Hebrew calendar.
Cycles of years
Every 247 years, or 13 cycles of 19 years, form a period known as an iggul, or the Iggul of Rabbi Nahshon. This period is notable in that the precise details of the calendar almost always (but not always) repeat over this period. This occurs because the molad interval (the average length of a Hebrew month) is 29.530594 days, which over 247 years results in a total of 90215.965 days. This is almost exactly 90216 days – a whole number and multiple of 7 (equalling the days of the week). So over 247 years, not only does the 19-year leap year cycle repeat itself, but the days of the week (and thus the days of Rosh Hashanah and the year length) typically repeat themselves.