a month: a unit of time of about thirty days, reckoned in the biblical world from one new moon to the next
In the New Testament this word usually answers the question "how long?" A stretch of time is measured out in months: Elizabeth kept herself in seclusion for five, Paul taught in one place for eighteen, and the drought in Elijah's day lasted three years and six months. In Revelation the same counting turns symbolic, with locust-torment running five months and a hostile power holding sway for forty-two.
The word can also fix a point on the calendar rather than measure a span. "In the sixth month" marks how far along Elizabeth's pregnancy had come, and the tree of life yields its fruit "every month." Behind all of this stands the moon: a biblical month ran from one new moon to the next, roughly twenty-nine or thirty days, so the word is bound up with the lunar cycle. The Hebrew counterpart חֹדֶשׁ makes the link plain, since it means both "month" and "new moon."
Because months were counted by the moon, they doubled as religious markers. When Paul warns the Galatians that they "observe days and months and seasons and years," the "months" are the recurring new-moon celebrations, close kin to the Greek word for that festival (νουμηνία).