Islamic Calendar
The Islamic calendar is based upon the lunar month; the period between one full moon and the next. Each year consists of 12 lunar months, making the year between 355 and 354 days in length. As a this is roughly 11 days shorter than the solar year, the months of the year drift between the seasons by 11 days each year, with each month returning to its original position after 33 Islamic years. The lunar calendar is principally used to give the dates of celebrations, ceremonies and the holy month of Ramadan. Ramadan begins with the first sighting of the crescent of a new moon. However there is some controversy among scholars as to whether the moon must actually be visible before the month begins or whether an astrological calculation can be made if the moon is obscured by cloud.
Moon Phase and Birthdate
Certain 'pagan' or astrological belief systems maintain that the phase of the moon in the sky when a person was born influences the character of the person. Those born under a new or waxing moon are seen as having outgoing, joyous, youthful personalities while those born under the waning moon are seen as more contemplative, thoughtful and calm. However there is no verifiable evidence for these claims and continued faith in such claims is regarded in the scientific community as a result of a cognitive bias.
Lunar Effect
Lunar effect refers to the pseudo scientific belief that the phases of the moon influence human behavior. In particular it suggests that the full moon causes an increase in delinquent behavior or exacerbates psychological illness. Though there is a lack of reliable evidence for this, researchers Sallie Baxendale Jennifer Fisher of University College, London have shown in their 2008 study "Moonstruck? The effect of the lunar cycle on seizures" that there may be a link between an increased incidence of epileptic seizures and the increase in nocturnal illumination caused by the full moon.
The Full Moon in Therevada Buddhism
Therevada Buddhism also operates according to a calendar with a lunar basis, though it does not precisely follow the phases of the moon as in Islam and is adjusted to match the solar year. In Therevada countries, various holy days occur on or around the full moon of certain lunar months. These days are used to commemorate particular events in the Buddha's life such as his birth, awakening, and death which is celebrated at Vesak during the first full moon of May.