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. Although there is no fixed day to perform Satyanarayan Puja but doing it during Purnima or Pournamiis considered highly auspicious.
Devotees should observe the fast on Puja day. The Puja can be done in the morning as well as in the evening. However performing Satyanarayan Puja in the evening is considered more appropriate as devotees can break the fast with Prasadam in the evening.