How the date is fixed
The Shukla Shashthi of every lunar month. 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†. Dark-half entries carry both month names: purnimanta (North India) and amanta.
About
The sixth tithi of every bright half is kept for Murugan (Skanda) in Tamil Nadu and across South India. The day on which Shashthi joins Panchami is preferred: if Shashthi begins between sunrise and sunset, that day is taken; if it begins at night, the next day. The fast can therefore fall on the civil day of Panchami. The Shashthi of amanta Kartika (Tamil Aippasi) is the annual Skanda Shashthi with Soorasamharam.
How it is observed
A fast from sunrise, broken in the evening after the worship of Murugan, with the Kanda Shashti Kavacham recited.