Sashti Fasting, or Shasti Vritham, is dedicated to Hindu God Muruga and is observed on the sixth day of the Shukla Paksha, or waxing phase of moon, in a Hindu lunar calendar. Muruga is also known as Kartik, Skanda, Subrahmanya and Shanmugha. There is an interesting story behind observing the Sashti Fasting.
One of the most important divine acts associated with Lord Muruga is the annihilation of demons led by Surapadma. This divine act to restore dharma is known as Soorasamharam.
Demon Surapadma had numerous powers and one of it was to make himself and the one fighting him invisible. He employed this power when he was engaged in a fight with Muruga. Goddess Parvati and other devas who were unable to see Muruga due to this demonic influence observed a fast. Finally Muruga appeared before them on the next day after annihilating demon Surapadma. Goddess Parvati and other devas broke their fast. It is believed that devotees started observing Sashti fasting after this incident.