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 re-interpreted as the Hebrew word for bitter, alluding to the fact that the month has no holidays or fasts.
7 Marcheshvan – V'tein Tal u-Matar ("Deliver Dew and Rain"), a prayer, is added to the Shemoneh Esrei prayers in Israel. If no rain has fallen by the 17th of the month, special prayers are added for rain 
Bahab – According to most minhagim, on the first Sabbath of Cheshvan, a prayer is recited on behalf of all those who are going to fast on Bahab. Bahab, or in Hebrew בהב stands for 2, 5, 2, which means Monday, Thursday, and Monday. On the Monday, Thursday, and second Monday after the Sabbath, the minhag is to fast and/or recite penitential prayers called Selichot. According to Minhag Ashkenaz, the second Monday of Bahab is the Monday before Rosh ChodeshKislev, the Thursday is the Thursday preceding that, the first Monday is the Monday preceding that, and the Sabbath, in which the prayer is recited, is the Sabbath preceding that. Bahab is also observed at the beginning of Iyar.