Shri Satyanarayan Puja is performed to seek blessing of God Narayan which is one of the forms of Lord Vishnu. The Lord in this form is considered an embodiment of truth.
Purnima means full moon day. Purnima occurs only once a month and considered auspicious by Hindus. Many devotees observe fast on this day from sunrise to sunset or moonrise.
Purnima or the full moon day is one of the most important times in the Hindu calendar.