Paksha (lunar fortnight)
A paksha is a lunar fortnight of 15 tithis. Shukla paksha is the bright half, when the Moon waxes from new to full; Krishna paksha is the dark half, when it wanes from full back to new.
Each lunar month has two pakshas. Shukla paksha, also called sudi, runs from the day after Amavasya to Purnima as the Moon grows brighter. Krishna paksha, called vadi or badi, runs from the day after Purnima to Amavasya as it fades. Dates are written with the paksha and tithi together, such as Shukla Ekadashi or Krishna Ashtami.
Shukla paksha is generally preferred for beginnings that should grow, such as weddings, griha pravesh and new ventures, because the Moon is gaining light. Krishna paksha is traditionally linked with ancestors, introspection and completion, and Pitru Paksha falls within it.
The dark half is not unlucky. Some of the best-loved festivals fall in Krishna paksha, including Janmashtami, Diwali and Maha Shivaratri, and its early days are still used for many muhurats. Note that a Krishna paksha’s month name differs between amanta and purnimanta calendars, though the day itself is the same.