It depends on your practice. Some people follow the moon cycles and feel this aids them with casting. Generally the waxing moon would be cast when you are looking for something to grow (ie. wealth, love, confidence, pregnancy) the waning moon is often used for diminishing or cleansing (weight loss, problems/burden, banishing)
But it is specific to what path and practice you follow. Some use days of the week others tides or specific hours. It all comes down to your spell (what you want to put into it) and your intentions. If you strongly believe the moon cycle will help "power" your spell more, than you would follow that belief.
Spell casting is very personal and so it can be conformed to your own beliefs. Once you start casting you get a sense of what works and what does not and eventually you find your path and from there you develop it. Some people use herbs and crystals while others use candles. Some don't even chant or say a spell out loud. It really is up to you and how you wrote your spell out.
Re: Best Days to Cast Spells By: Lark Moderator / Knowledgeable
Post # 6 Mar 16, 2020
Here's some suggestions for the best days of the week for specific types of spells. https://witcheslore.com/bookofshadows/rituals-spell-casting/time-for-magic-which-day-of-the-week-to-cast-your-spell/32586/
Monday is associated with the moon and is best for divination and water associated magick. Tuesday is associated with Mars and can be good for any fiery or sexual spell. Wednesday is associated with mercury and communication. Thursday is associated with Jupiter and magick associated with wealth and abundance. Friday is associated with Venus and is best for love and beauty magick. Saturday is associated with Saturn and is the best day to complete an important ritual. Sunday is associated with the Sun and fame and wealth. Also look at the moon phases as you make not want to do a ritual drawing things to you during the waning moon, or dark magick during the waxing moon. The full moon is an added boost.
In Tibet, there are certain sacred days of the year that are very "energized" by moon phases, eclipses, and solstices. In this case, these would be the 8th day after the new moon (sometimes referred to as Tara or Medicine Buddha Day), the 10th day after the new moon (Guru Rinpoche Day), sometimes the 14th day (especially with the Medicine Buddha), the 15th day after the new moon (aka. the Full Moon, a very powerful day that one perform rites such as those of the Pureland Buddha Amitabha or the Medicine Buddha or any other enlightened deity), the 25th day after the new moon (Dakini day, a powerful day of enlightened feminine deities like the peaceful Tara, longevity Namgyalma, the mighty Dhvajagrakeyura, lineage matriarchs like Machig Labdron or Yeshe Tsogyal, the wrathful Vajravarahi, etc.), and the 31st day (aka. the new moon, which is Shakyamuni Buddha's day and a very powerful one as well). Such are the powerful phase days according to the Tibetan tradition. Of course, the power is very, very high on new years, solstices, or eclipses, these days are especially important.