It is important to start meditating at least 1 day before a new moon or full moon day. It is said to be a day where the heaven, earth and the hell gets in perfect alignment. (The alignment of the sun, moon and earth.)
Past karma is also said to manifest on a new moon and full moon day, it manifests one’s karma and also deepest desire. During such days, if one is filled with joy and merits, such energies will get magnified tremendously and vice versa, if one is accumulated with heavy karma, the negative energies will also be magnified significantly.
It is during a new moon and full moon day, a negative karmic cycle get a chance to be shifted into a positive cycle.
And it is on a full moon day all forms of spirits can be aligned to a person. If a person has accumulation of many past lives creditors, it will also be on the full moon day such spirits will come to seek karmic debt. Thus, it is believed that practice towards merit makings will help alleviate or totally eliminate past karmic issues.
If one has accumulated more merits than demerits, the accumulation of spirit guides and deities will give spiritual guidance to help one advance onto the spiritual path in a positive way.