Month (Sax. mona, the moon), a period of time defined by one revolution of the moon around the earth, and hence equal to 29 days, 12 hours, 44 minutes, and 3 seconds. This division of time, the lunar month, was used by the Chaldeans and Egyptians, and is still by the Jews, Turks, and many uncivilized nations, as the most distinctly marked period of the year. But if the year be made to comprise 12 of these months, the seasons will soon be found to fall back from those months to which they originally belonged, so that in 34 years each month would fall in each of the seasons. The civil year is divided into 12 months of an average length of 30 days, 10 hours, and 30 minutes. But these (called calendar months) are not equal, some (April, June, September, and November) consisting of 30, and the remainder of 31 days, except February, to which only 28 days are assigned, with the addition in leap years (every fourth year) of one more day. In popular language a month is often understood to be four weeks. This is even laid down by Blackstone as the legal definition of the term, so that a lease for 12 months is only for 48 weeks; but the expression "a twelvemonth " has been legally held to mean a solar year.

In ecclesiastical and commercial matters, however, month always means a calendar month; and this is generally the legal meaning of the word in all relations in the United States.