Sharad - The season marking the end of monsoon and the beginning of winter.
Poonam - Full moon day.
Aaso Sud 15.
On this day, when the moon is at its brightest in the year, Lord Swaminarayan regularly played Raas (traditional folk dance) with other Satsangi's. The kirtan 'Juo Juo Sahelio Aaj Rasio Raas Rame...' depicts one such amazing occasion in Panchala, when Bhagwan Swaminarayan assumed multiple forms and played raas with all the Satsangi's under the light of the moon.
Also on Sharad Poonam Lord Krishna played Raas with the Gopi's in Vrundavan. Like the moon glows in the sky, Vrundavan was glowing with Lord Krishna's presence and the air was filled with the sweet sound originating from his flute.
On hearing this sweet sound the Gopi's immediately abandoned their home chores and rushed towards where this sound was emanating from. This showed how devoted they were to Lord Krishna. Their devotion and love for Lord Krishna was so great that the Gopi's commenced playing raas around Him. Lord Krishna and Radhikaji played raas in the middle and eventually He assumed as many forms as there were Gopi's and played raas with each and every one of them.
The essence of this event is that our devotion for God should be like the love of the Gopi's to Lord Krishna.
Traditionally dudh pakh (rice pudding) and puri (fried chappati's made from wheatflour) is offered to God and taken as prasad. Raas is played with great fervor in the Temples, in the presence of the full moon.
Reference: Ved Vyas Shrimad Bhagwat 10 skandh.