The Silver Moon is a special astrological event that occurs every 100 years. During this special event, the most recent descendant of Hansel is bestowed with the powers of the Golden Child.

We first learn of this event in Rise of the Snow Queen and it's also briefly mentioned in Queen of Sands. It is unknown when this event started, but it can be assumed it's existed since the Moon Goddess's creation.  

It is during this event that the Snow Queen searched for the Golden Child in the Swiss Alps to help her son. She kidnapped Gerda, whose powers were shown during the Silver Moon.