In general, Jewish-calendar months alternate in length: odd-numbered months have thirty days each and even-numbered months have twenty-nine. However, it is sometimes necessary for a year to have one fewer or one more day than would thus be allowed, so a day is added or removed from the calendar. Specifically, in nearly half the years, Marcheshvan, though the eighth month, has thirty days; in half the remaining years, Kislev, though the ninth month, has but twenty-nine days.
Why were Marcheshvan and Kislev, specifically and rather than any other months, chosen to vary in length?