New moon

 In astronomy, new moon is the first phase of the Moon, when it lies closest to the Sun in the sky as seen from the Earth. More precisely, it is the instant when the Moon and the Sun have the same ecliptical longitude. The Moon is not normally visible at this time except when it is seen in silhouette during a solar eclipse. See the article on phases of the Moon for further details.

The original meaning of the phrase new moon was the first visible crescent of the Moon, after conjunction with the Sun. This takes place over the western horizon in a brief period between sunset and moonset, and therefore the precise time and even the date of the appearance of the new moon by this definition will be influenced by the geographical location of the observer. The astronomical new moon, sometimes known as the dark moon to avoid confusion, occurs by definition at the moment of conjunction in ecliptical longitude with the Sun, when the Moon is invisible from the Earth. This moment is unique and does not depend on location, and in certain circumstances it coincides with a solar eclipse.

The new moon in its original meaning of first crescent marks the beginning of the month in lunar calendars such as the Muslim calendar, and in lunisolar calendars such as the Hebrew calendar, Hindu calendars, and Buddhist calendar. But in the Chinese calendar, the beginning of the month is marked by the dark moon.

Although the new moon is typically depicted as a black circle, its actual phase is a very thin crescent, because the moon does not pass directly in front of the sun (except during an eclipse). On July 8, 2013, French astrophotographer Thierry Legault successfully photographed the new moon, although the crescent itself was not visible to the unaided eye.

    The new moon is quite significant in Hindu calendar. People generally wait for new moon to start new works. Waxing period of moon is considered good for all good works. Fifteen Moon dates each for waxing and waning period are there. Fifteen dates are classified in five categories, namely Nanda, Bhadra, Jaya, Rikta and Purna and three rotations of these five categories are there. The category rotation starts from first date of moon ending at fifth date and then starting at sixth date and so on. Nanda dates come on First, Sixth and Eleventh moon date, same can be known about others. Nanda dates are good for auspicious works. Bhadra dates can be good for works related with community, social, family, friends. Jaya dates are good where we need to deal with some conflict. Rikta dates are not considered much good and do good for works related with cruelty. Purna dates are good for every work. The first day of the Lunar Hindu calender starts the day after the no moon day(Amavasya). Hindu astrology considers amavasya as powerful, either good or bad. The Hindu epic, Mahabharatha states that the Kurukshetra war started on the Amavasya day that too on a Tudesay (Mangalvaar, day of the week named after Mars).

    The new moon is the beginning of the month in the Chinese calendar. Some Buddhist Chinese keep a vegetarian diet on the new moon and full moon each month.
    The new moon signifies the start of every Jewish month, and is considered an important date and minor holiday in the Hebrew calendar. The modern form of the calendar is a rule-based lunisolar calendar, akin to the Chinese calendar, measuring months defined in lunar cycles as well as years measured in solar cycles, and distinct from the purely lunar
    The new moon is also important in astrology, as is the full moon.

Source

Wikipedia:New moon