First of all it refers to the fact that the circumstances of that day (the coincidence with Purnama and the dominant presence of highly positive cosmic energy that day) are most auspicious for cleansing oneself and becoming aware of the existence of one's higher self who strives for a life based on dharma versus the dominant ego who strives for adharma.  Secondly it points to the fact that if the higher self would be dominant over the ego, Bali would be like the world of the gods - or rather people (mankind) would be like gods, living life as it is supposed to be, as co-creators, with the awareness of a divine (higher) consciousness. And this is exact the intention of Galungan, to create awareness of the dualty of intentions of the higher self and the ego, and to try to reach one's higher consciousness. If one succeeds, the higher self will be dominant over the ego and the inner battle between dharma and adharma is over.