There are eight Moon phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent. However, it is not as simple as dividing 29.5 by eight to find out how long each phase lasts. The fraction by which the moon is illuminated is steadily increasing or decreasing at all times.
The primary phases of the Moon cycle are the new moon, the first quarter, the full moon and the last quarter. The dates and times of these phrases -- which can be calculated and documented on calendars -- change from month to month according to the positions of the Sun and the Moon. The crescent and gibbous phases are known as intermediate phases.
The phases of the Moon are connected to the position of the Moon in relation to the Sun. While the new moon is dictated by the close proximity of the Sun to the Moon, the full moon occurs when they are at opposite positions in the sky. The first and last quarters happen when the Moon has traveled one-and three-quarters way around its orbit.
As the phase cycles of the Moon are almost always shorter than a full calendar month, the phase of moon that occurs at the start of the month usually occurs again before the end of the month. As a result, every 2.7 years, the full moon phase occurs twice in the same month. The second of these full moons is called a "Blue Moon." Although 2.7 years can hardly be considered an irregular occurrence, this is where the phrase "Once in a blue moon" -- meaning something that happens extremely rarely -- originates. The beginning of the Islamic month is marked by the first appearance of the waxing crescent moon. In some cultures, it is considered unlucky to see the new moon through glass.