How the date is fixed
Skanda Shashthi falls on the sixth day of the bright half of the lunar month Kartika (Shukla Shashthi). The date is the day on which the tithi begins, if it begins between sunrise and sunset (that day it is joined with the tithi before it); if it begins at night, the next day, when it is running at sunrise†.
About
Skanda Shashthi, or Kanda Shashti, is the sixth tithi of the bright half of the lunar month that falls in the Tamil solar month Aippasi, which is amanta Kartika, in October or November. It closes a six-day observance for Murugan (Skanda) and is the day of Soorasamharam, the enactment of his victory over Surapadman, held at Tiruchendur and other Murugan temples. Following the Tiruchendur practice, the day on which Shashthi joins Panchami is preferred: if Shashthi begins between sunrise and sunset, that day is taken, otherwise the next. The same rule fixes the monthly Skanda Shashthi.
How it is observed
A six-day fast, the Soorasamharam enactment at dusk, and the Tirukalyanam of Murugan the next day.