The auspicious days for Muruhan: Fridays, the constellation of Kaarthihai and the Sashti thithi. As Lord Muruhan is the presiding deity for Mars, Tuesday is also auspicious to HIM. Among these, it is the Sashti fast,specially the Kantha Sashti fast, that is discussed here.
Sashti: Sashti is based on thithi.The moon grows layer by layer (kalai) after the new moon and becomes full moon; then it fades layer by layer and ends up as a new moon. The fourteen days between the new moon and full moon are known as fourteen thithis; a thithi can be longer or shorter than a day. Sanskrit numerals are used to name the thithi.
Sashti means six; hence there will be two sashti thithis every month. The sashti thithi that occurs after the new moon is denoted as “sukkila patcha sashti” or “poorva patcha sashti”; in other words it is the sashti that occurs on the brighter side of the moon, i.e., during the waxing moon. Although all the sashti during the waxing moon are auspicious for Lord Muruhan, the sukkila patcha sashti that occurs in the Tamil month, Aippasi, (mid-October to mid- November) is most auspicious. The six days, after the new moon in the month of Aippasi, are collectively allocated for the Kantha Sashti Fast. Devotees of Muruhan observe fast during the six days.