Blue moon

From Charmed

A moon phase that traditionally loosenes the bonds of magic, on the three days when the blue moon rises, it can wreak havok with all magical forces, causing witches powers to become eratic and unpedictible or even inneffective, however, this is only temporary and only lasts while the blue moon does. While some witches may find the ties between themselves and their powers lessened, the Charmed Ones will have an added worry, because not only will the ties between each sister and her power lessen, each sister's bond to each other will be weakened as well, weakining the power of three as a whole, which makes it easier for evil to attempt to destroy it. It is imperative that the power of three be guarded on this day.