This is an excerpt from the Hari Bhakti Vilasa: Sri Janmastami Vrata Fifteenth Vilasa, Volume Two beginning with text 249
Sri Krishna said: O Yudhishthira, after I had killed the demon Kamsa, Devaki embraced Me, placed Me on her lap, and wept. When the wrestling was over she stood up in the grandstand. The Kukuras and Andhakas praised her. Her affectionate kinsmen and their wives surrounded her. Vasudeva came, embraced me, said, "Son! Son!" and wept with a father's love. His eyes filled with tears, he embraced Balarama and Me. In a choked voice he said, “Today my birth has born fruit. Today my life is lived well. Now I have met my sons, the two glories of the Yadu dynasty.” O king, seeing My parents' happiness, everyone else became happy. Bowing before Me, everyone spoke these words: “Today we are happy. Today Krishna is born. Today the demon Kamsa has fallen in a wrestling match. O Lord Krishna, please glance at the people in this festival. Please give Your mercy to them and to others also.O Krishna, O Lord who removes all sufferings, please give to us the day when Devaki gave birth to You. We will celebrate a festival in Your honor. O Krishna, please be kind to Your sincere devotees.” Filled with wonder to hear the people speak these words, and the hairs of his body now erect in ecstasy, Vasudeva glanced at Balarama and said: “It should be so. Please tell the people.” O son of Pritha, then, on My father's order, I revealed the vow of Janmastami to the people of Mathura. The brahmanas, kshatriyas, vaishyas, shudras, and all others who are pious, from the age of eight years and older, should observe Janmastami. (ekadasi is followed at the same age as well.) O people, during the monsoon season, on the ashtami of the dark fortnight (krishna-paksha, of the month of Bhadra, when the sun was in Leo and the moon and the star Rohini were both in Taurus, in the middle of the night I was born as the son of Vasudeva and Devaki. This day will be known in this world as the vow of Janmastami. On this day celebrate a great festival honoring both Me and the goddess. First this festival will be celebrated in Mathura. Later it will become famous in the whole world. Hearing of this festival, the people began to observe it. May there be peace amongst them. May they be happy. May they have good health. This festival should be observed every year, for that is pleasing to the Lord. The scriptures also describe some offenses in its observance.
In the Skanda Purana it is said:
Prahlada and other great kings very faithfully observe the auspicious vow of Janmastami, which is dear to Lord Krishna. Their purpose is to please Lord Vishnu
It was nice noticing Lord Sri Krishna’s prediction that this festival would be celebrated all over the world. Hare Krishna