First let me address the information about the moon phases which you had incorrect and add some other information while im at it. (quick brief skim of info)
The wanning gibbous is when the moon is moving into its last quarter, its a reducing of light from the moon at that point in time etc, thus it promotes the use of energy to "rid or remove" things.
Banishments, Removal of hexes/jinxes, cleansings are a few of the many things supported by thewaningmoon.
Thewaninggibbous is the coming of the dark moon, a time of rest in one aspect
The waxing gibbous is the moon that increases gradually into a full moon, thus promoting in energy wise the increase of things.
Money spells,healing, certain divinatory arts
It is said that if you cut your hair during this phase it, will easily grow back. This is cause of the various celestial energies that bombard our physical/spiritual bodies on a daily basis
This moon allows for you enter into all various energies thus containing no waxing/wanning, all spells can be easily cast on this day. it is also said spells are more powerful because of the full frequency of the moon's energy which makes itself present.
Eclipses are rare vents often combining both sun and moon energy into one creates a massive amount of energy. Eclipses are extremely powerful and actually empower spells greater than that of a full moon ( just like an astrological event)
The new moon, brings a "new" vibration into being. It promotes any thing new, good for changing location, getting new things going on a new path things of that nature. It is just like a full moon in a sense of frequency but just the vibration is noomnipotent.
The dark of the moon is a time when the sky looks black and there is no visible moon in sight. The energy goes to a low point but inverts a bit. Negative things may thrive on this but there are allot of positive forces that do come out of the dark moon. Even in advanced blood magick, you have to understand the dark goddesses of transformation and many women in the field may use their menstral blood.
Now as for your question, yes you can't cast spells at any time, but as a beginner you need all the energy you can get so the moon is one good way of dealing with it Remember this is just a quick skim you can get more lengthy explainations on the web.