month: [OE] In ancient times the passing of time was recorded by noting the revolutions of the moon. Consequently prehistoric Indo-European had a single word, *mēnes-, which denoted both ‘moon’ and ‘month’. The Romance languages retain it only for ‘month’: Latin mēnsis (source of English menstrual) has given French mois, Italian mese, and Spanish mes. The Germanic languages, however, have kept both, distinguishing them by different forms. In the case of ‘month’, the Germanic word was *mǣnōth, which has differentiated into German monat, Dutch maand, Swedish månad, Danish maaned, and English month. => menstrual, moon
Old English monað, from Proto-Germanic *menoth- (cognates: Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), related to *menon- "moon" (see moon (n.); the month was calculated from lunar phases). Its cognates mean only "month" in the Romance languages, but in Germanic generally continue to do double duty. Phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months, but never used literally).