After Narayana rose from Yog Nidra, he went to his Guru, Mahadeva. Seeing each other, they embraced each other. During the act, the bael leaf of Shiva Ji stuck to Narayana’s robes, and the same happened with the basil leaf of Narayana.
This makes us remember the verse,
Guru Brahma Gurur Vishnu, Guru Devo Maheshwarah.
Guru Sakshat Param Brahma Tasmai Shree Guruvey Namah.
There is no distinction between Narayana and Shiva. They are the same Eternity. Names might vary. The day when Narayana (Hari) and Shiva (Har) met was Hari Har Milan. Even, today we celebrate this auspicious day. People offer bael leaves to Narayana and basil leaves to Shiva to mark this day. Rituals are futile. Only a fertile and pure mind is inevitable to walk on the path to spirituality.
People light lamps on the day of amavasya, the day when mortals celebrated the return of Rama to Ayodhya; The Deepawali. Then, after eleven days, Narayana rose from Yoga Nidra on the day of Dev Uthayana Ekadashi. Then, on the second day from the Ekadashi, Narayana went to meet Shiva. That day is celebrated as Deva Diwali.