Pratipada, the First Day of the Fortnight

Pratipada is the first tithi, the opening lunar day of the fortnight. A tithi in the Hindu calendar is not a solar day but the time the Moon takes to gain twelve degrees of longitude on the Sun, so the thirty tithis divide one synodic month into two fortnights (paksha) of fifteen each. This Pratipada falls in the Shukla or bright fortnight, the waxing half that begins just after the new moon. In the traditional fivefold grouping of tithis it is a Nanda tithi, one of the joyful days, and its presiding deity is commonly given as Agni or Brahma. Being the start of the waxing Moon, it is read in the panchanga as favourable for beginnings and fresh undertakings. These are the calendar tradition's conventions; the astronomical basis is simply the widening angle between Moon and Sun after conjunction.

A tithi's real-world duration is never fixed: because the Moon's angular speed varies between roughly eleven and fifteen degrees a day over its elliptical orbit while the Sun's varies far less, the twelve-degree gain that defines a tithi can take as little as about nineteen hours or as long as about twenty-six, so Pratipada on one date is not the same length of clock time as Pratipada a few months later. This variability produces two calendrical phenomena the panchanga must account for: tithi kshaya, when a tithi begins and ends between one sunrise and the next and so never governs a civil day at all and is skipped in the naming of dates, and tithi vriddhi, when a tithi is long enough to span two consecutive sunrises and so the same tithi name is given to two days running. Both are ordinary arithmetic consequences of the Moon's uneven motion, not irregularities in the count itself.