This festival is celebrated in reverence of Kartikeya on Chaitra Shukla Shashthi (sixth day in the bright fortnight of the Hindu lunar month Chaitra), Bhagwan Vishnu and Bhagwan Shiv is believed most superior Gods and Mata Uma (wife of Bhagwan Shiv) is the most superior Goddess. Katikeya is son of Lord Shiva and Uma. Once there was constant long war between Gods and demons. Demons were proving stronger as they had very brave and able commanders like Hiranyakashyapu, Himyaksha, Bhimaksha, etc. but the Gods were depending on their single commander Indra. Guru Brihaspati (the guru of Devtas (Gods)) advised the Gods to arrange a competent commander. They all went to Lord Brahma, who told them that in the commandership of Shiva’s son, demons could be defeated. He suggested to them to request Shiva capable commander. They all went to Bhagwan (Lord) Shiv and prayed for a brave and capable commander who could defeat the demons. Bhagwan Shiv gave assurance to them; that he would arrange such a commander. Then Bhagwan Shiv inspired his wife Uma and by their union, a brilliant and brave son, Kartikeya born on this day. Six mothers nourished him. His personality and strength were extraordinary. Bhagwan Shiv sent Kartikeya to be commander of the army of Devtas (Gods) along with two attendants Shakh and Vaisakh. The Gods invoked him as Kumar Kartikeya. Soon after this, Kartikeya developed a huge form. In his commandership Gods defeated the demons. People observe fast to invoke Kartikeya for long life and fulfillment of their desires on this day.
Aum Namah Shivaye
Source: Book: Hindu Festivals By Shashi Kant Kaushik