This is a wonderful question and one of the most personal as well. Many factors denote when to cast a spell for different witches. No two witches are exactly the same in practice.
Some of the factors are casters need, Time of day, Day of Week, Month, Moonphase, Astrological movement, Sun Phase and so much more.
Most of all, when do you feel you need to cast the spell?
Yes, casting when the spell alines with one of the many factors does indeed make them stronger. But we cannot always wait. Think of it like this. you have a cut on your hand are you going to wait till a full moon to put a bandaid on it just because it will heal faster than or are you going to bandage it now because of the need there?