Paksha

Paksha (पक्ष) refers to a fortnight or a lunar phase in a month of the Hindu lunar calendar.

Literally meaning "side", a paksha is the period either side of the purnima (full moon day). A lunar month in the Hindu calendar has two fortnights, and begins with the amavasya (new moon). The lunar days are called tithis; each month has 30 tithis, which may vary from 20 – 27 hours. A paksha has 15 tithis, which are calculated by a 12 degree motion of the moon. The first fortnight between the new moon day and the full moon day is called the Gaura Paksha or Shukla Paksha (lit. 'white/bright/golden side'), the period of the brightening moon (waxing moon), and the second fortnight of the month is called the Vadhya Paksha or Krishna Paksha (lit. 'dark/black side'), the period of the fading moon (waning moon). The Neemuch calendar begin a new lunar month from first day of Krishna Paksha, while the Gujarat calendar begins a new lunar month from the first day of Shukla Paksha.

Shukla Paksha
Shukla Paksha refers to the bright lunar fortnight or waxing moon in the Hindu calendar. Shukla (शुक्ल) is a Sanskrit word for "white" or "bright". The Shukla Paksha is a period of 15 days, which begins after the amavasya (new moon) day and culminating on purnima (full moon) day and is considered auspicious.

Numerous festivals are held during this period, including the Navaratri festivals, most importantly Chaitra Navaratri and Ashvin Navaratri.

Krishna Paksha
Krishna Paksha refers to the dark lunar fortnight or waning moon in the Hindu calendar. Krishna (कृष्ण) is a Sanskrit word for "dark". Krishna Paksha is a period of 15 days, which begins after purnima day (full moon), culminating on the amavasya (new moon). Krishna Paksha is considered inauspicious, as the moon loses light during this period.

Festivals during the Krishna Paksha are:

Other usages
In Vedic astrology when a person does a prasna (a question chart) and the planet Venus indicates the time period, the event referred to in the answer will happen in a pakṣa (fortnight) from the time the question was asked.