ಪಕ್ಷ: ಎರಡು ಪಕ್ಷಗಳು | Paksha: the two fortnights

A lunar month divides into two fortnights of fifteen tithis each, called paksha, literally a wing.

Shukla paksha is the bright fortnight, from the new moon to the full moon, while the moon is waxing. Krishna paksha is the dark fortnight, from the full moon to the new moon, while it wanes. Kannada and Telugu usually call the dark fortnight bahula paksha rather than krishna, and both terms mean the same thing.

What follows from it

The paksha is not decoration. It determines which of two tithis with the same number you are looking at: Ekadashi of the bright fortnight and Ekadashi of the dark fortnight are twelve or so days apart and carry different names and different observances. An almanac that gives a tithi without its paksha has given you half the information.

Most vratas occur in both fortnights but are weighted differently. Ekadashi is kept twice a month by those who keep it strictly, once by many. Pradosha likewise falls on the thirteenth of each fortnight. Sankashti Chaturthi belongs to the dark fortnight and Vinayaka Chaturthi to the bright, and they are distinct observances despite sharing a tithi number.

Bright work and dark work

As a broad rule, the waxing fortnight is preferred for beginnings: weddings, house entry, starting a business, beginning study. The reasoning given is that the moon is gaining light, and a venture begun then is held to grow with it. The waning fortnight is preferred for things that are meant to diminish or conclude, and for rites for the departed. Amavasya in particular is the day for tarpana, and the Mahalaya Amavasya of Bhadrapada closes the fortnight given to the ancestors.

This is a preference rather than a prohibition, and plenty of observances fall in the dark fortnight, Janmashtami and Maha Shivaratri among the largest.

Where the month begins

There are two conventions for where a month starts, and they differ precisely by a paksha. In the amanta reckoning, followed in Karnataka, Maharashtra, Andhra Pradesh, Telangana, Tamil Nadu and Gujarat, the month runs new moon to new moon, so the bright fortnight comes first. In the purnimanta reckoning of much of North India, it runs full moon to full moon, so the dark fortnight comes first.

The consequence is that during the dark fortnight the two systems give the same days different month names, one month apart. The tithi is identical; only the label differs. Anudina shows the amanta name by default, as Karnataka does.