The month was originally defined as the time between one new moon to the next, a period now called the synodic month. In the modern calendar, however, there are 12 months in a year, each having a fixed number of days in order to sum to 365 (or 366 in a leap year).

Prior to 46 BC, the Roman calendar, or what has been reconstructed of it, is described as a "mess." The Roman calendar originally started the year with the vernal equinox and consisted of 10 months (Martius, Aprilis, Maius, Junius, Quntilis, Sextilis, September, October, November, and December) having a total of 304 days. The numbers still embedded in the last four months of the year are the fossil of this (September, October, November, and December, contain the Latin roots for the numerals seven, eight, nine, and ten, but now fall on the ninth, tenth, eleventh and twelfth months of the year). The 304 days were followed by an unnamed, unnumbered period in winter. The Roman emperor Numa Pompilius (715-673 BC) introduced February and January between January and March, increasing the length of the year to 354 or 355 days. Then in 450 BC, February was moved to its current position.

The number of days allocated to each month follows a complicated scheme in which most months have 30 or 31 days, but the details have been determined by historical precedent instead of logical considerations.

There are also several other kinds of months which are of particular importance to astronomers.

The synodic month is the mean interval between conjunctions of the Moon and Sun, corresponds to the cycle of lunar phases. The anomalistic month is the time between perigee passages. The Draconic month is the time between one node passage to the next. The sidereal month is the time between maximum elevations of a fixed star as seen from the Moon. The tropical month is the period from one lunar equinox to the next.