Tithi

In vedic timekeeping, a tithi (also spelled thithi) is a lunar day, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.

There are 30 tithis in each lunar month, named as follows:

 Krsna paksa                        Gaura paksa
 (dark fortnight)                   (bright fortnight)
 1   Pratipat                       1   Pratipat
 2   Dvitiya                        2   Dvitiya
 3   Trtiya                         3   Trtiya
 4   Caturthi                       4   Caturthi
 5   Pancami                        5   Pancami
 6   Sasti                          6   Sasti
 7   Saptami                        7   Saptami
 8   Astami                         8   Astami
 9   Navami                         9   Navami
10   Dasami                        10   Dasami
11   Ekadasi                       11   Ekadasi
12   Dvadasi                       12   Dvadasi
13   Trayodasi                     13   Trayodasi
14   Caturdasi                     14   Caturdasi
15   Amavasya (new moon)           15   Purnima (full moon)

Ekadasi, the eleventh day of each lunar fortnight, has special religious significance in Hinduism and Jainism—usually observed by fasting.