The spell you do depends on the moon phase: full, new, waxing or waning. Waning is from full to new and waxing is from new to full.
New moons are times for cleansing and new beginnings, waxing moons are times for burgeoning energy and setting your sights on new goals or projects, full moons are when Witches' may feel at their strongest: this is a time for casting spells outward and celebrating the gifts of the Goddess. Waning moons is good for things you have to let go of and for breaking bad habits. The main three moon phases: waxing, full and waning show the phases of the Goddess too, as She is associated with the moon. It shows Her as a maiden, mother and crone.
There are thirteen full moons a year.