Marcheshvan (Hebrew: מַרְחֶשְׁוָן, Standard Marḥešvan Tiberian Marḥešwān; from Akkadian waraḫsamnu, literally "eighth month"), sometimes shortened to Cheshvan (חֶשְׁוָן, Standard Ḥešvan Tiberian Ḥešwān), is the second month of the civil year (which starts on 1 Tishrei) and the eighth month of the ecclesiastical year (which starts on 1 Nisan) on the Hebrew calendar.
In a regular (kesidran) year Marcheshvan has 29 days, but because of the Rosh Hashanah postponement rules, in some years an additional day is added to Marcheshvan to make the year a "full" (maleh) year. Marcheshvan is an autumn month which occurs in October–November in the Gregorian calendar.
Given the Akkadian etymology, it seems likely the מ and the ו were switched at some point in time, since w-r-ḥ is the Semitic root for "moon" (and thus also "month"), and š-m-n is the Semitic root for "eight". Also, מ and ו are labials. Since then, the first two letters מַר (mar) have been reinterpreted as the Hebrew word for bitter, alluding to the fact that the month has no holidays or fasts.