This day is celebrated as a festival in reverence to the birth of Guru Nanak born on this day in 1526. Guru Nanak, born in a Hindu Ksatri family, came from a family of merchants. From the very beginning of the life, his outlook was saintly. He had no love for worldly affairs. In his instinct for religious curiosity, he went to Varanasi, got education in the Vedas and established the religion of Sikhs. As the time demanded, the preaching was mostly for the unification of the Hindu and the Muslims. "There is neither a Hindu, nor a Muslim: only man." (Guru Nanak) His two main disciples were Muslims. His preaching was fundamentally "Vedic" preached in rhythmic sweet popular couplet. Later this sect of Hindus was given a martial turn and was named "Sikhism".
A dip in holy rivers and a fast on this day is considered very auspicious. According to Mataseye Purana, Lord Vishnu incarnated as "Mataseye" on this day. This day has been recognized as most auspicious by gods Brahma, Vishnu, Siva, Angira and Aditeye. On this day God Siva killed the demon Tripurasur, as described under "Kartik Purnima".
Further Reading On Guru Nanak