Shashthi

Shashthi is the sixth tithi of the bright fortnight, a Nanda or joyful tithi in the fivefold grouping. Its presiding deity is Kartikeya, also called Skanda or Murugan, the war-god and son of Shiva, and the sixth tithi is his throughout the calendar. Skanda Shashthi, in the bright fortnight of the autumn month, is his major festival, especially in the Tamil tradition where Murugan is a central deity; the day commemorates his victory over the demon Surapadman. Around the sixth tithi the waxing Moon approaches its first quarter. In the panchanga the day is read as favourable, and its devotional weight rests on Skanda rather than on the general classification. These attributions belong to the calendar and festival tradition.

Murugan's especially strong identification with the sixth tithi in Tamil Shaiva tradition runs deep enough that Skanda Shashthi is observed as a six-day cycle of fasting culminating on the tithi itself, commemorating the god's mythological six-day war against the demon Surapadman before his final victory, so the festival's structure directly echoes the number that names the tithi. Because Kartikeya, Skanda and Murugan are regionally distinct emphases of what the wider Puranic tradition treats as a single deity, the sixth tithi's devotional colouring can differ noticeably between a South Indian and a North Indian panchanga even though the underlying tithi arithmetic, the Moon's twelve-degree gain on the Sun, is identical everywhere.

A tithi is not a day. It is an arc, the interval in which the Moon gains twelve degrees of longitude on the Sun, and the civil day merely borrows its name. Shashthi occupies the arc from 60 to 72 degrees of that separation. Since lunar motion quickens and slackens across the month, the arc is crossed in as little as about nineteen hours or as much as about twenty six. The panchanga settles the mismatch by naming each civil day for the tithi in force at its sunrise. A tithi shorter than the interval between two sunrises can therefore be skipped entirely, which the tradition calls a kshaya tithi, and one longer than that interval can carry its name over two civil days, an adhika or vriddhi tithi. Both occur in the ordinary course of a year, and both are why an observance can appear on two dates in the same almanac.

Each tithi is halved again, and each half carries a karana, so the first half of Shashthi carries Kaulava, the third, and the second Taitila, the fourth. Seven of the eleven karanas repeat, cycling eight times through the month, while four are fixed to particular half tithis at the month's opening and close.

The Moon is waxing through this arc, setting later each evening and carrying about 30 per cent of the disc lit. That figure is a consequence of the definition rather than an observation added to it: elongation fixes the phase, so naming the tithi already names how much of the disc is lit.