Triumph of Krishna over Narakasura ,with the help of Satyabhama ,is celebrated as Narakchaturdashi .Shrikrishna killed Narkasura who was like a big forest ,snatched his earrings and gave it back to Diti devi .There he saw Parijat tree in swarga.Parijata is a clestial tree whose flowers emanate a very pleasant aroma ,very dear to Lord. One must not use flowers ,that have been picked up from the ground, to the feet of Hari., but Parijata is exception.it can be offered in any form to Krishna,who fought with Indra and brought the tree to the earth.
A bath after applying oil on the body in the early hours of Narakchaturdashi ,washes off all the sins.Chaturdashi is a riktha tithi, any act which requires thorough riddance should be carried on riktha tithi. the other riktha tithis are , chaturthi,navami. chaturthi is chosen to get rid of obstacles.navami is used to get rid of instability. chaturdashi rids one from all sins.