What is the X in X-mas?
Old English monað, from Proto-Germanic *menoth- (cf. 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).
Among the Egyptians the month of thirty days each was in use long before the time of the Exodus, and formed the basis of their calculations. From the time of the institution of the Mosaic law the month among the Jews was lunar. The cycle of religious feasts depended on the moon. The commencement of a month was determined by the observation of the new moon. The number of months in the year was usually twelve (1 Kings 4:7; 1 Chr. 27:1-15); but every third year an additional month (ve-Adar) was inserted, so as to make the months coincide with the seasons. "The Hebrews and Phoenicians had no word for month save 'moon,' and only saved their calendar from becoming vague like that of the Moslems by the interpolation of an additional month. There is no evidence at all that they ever used a true solar year such as the Egyptians possessed. The latter had twelve months of thirty days and five epagomenac or odd days.", Palestine Quarterly, January 1889.
a measure of time corresponding or nearly corresponding to the length of time required by the Moon to revolve once around the Earth