Knowing the best days for casting spells is not as common as planning around the moon phases, but it can make a difference. Each day is traditionally associated with a planet, and it has its own influence on the magick you perform that day. If possible plan your casting of spells for the right day for some added energy boost.
Monday (the Moon) - family, kids, spirituality, psychic skills, dreams, female energy, fertility
Tuesday (Mars) - energy, action, independence, conquering obstacles, dealing with conflict
Wednesday (Mercury) - creativity, communication, knowledge, jobs, career, education
Thursday (Jupiter) - wealth, finances, business, legal matters
Friday (Venus) - love, romance, relationships, family, marriage, social status, sexuality
Saturday (Saturn) - protection, discipline, overcoming habits, banishing, binding, meditation
Sunday (the Sun) - general success, abundance, strength, accomplishment, happiness, health
The best days for casting spells is just a rough guideline. If it's not convenient to cast your spell on the exact day, don't fret about it. It's only one small piece of the overall ritual and isn't going to ruin the entire spell if you do it on the "wrong" day. But if you are able to schedule your spells to your liking, this is a good place to start.
For a larger collection of these kinds of spell correspondences, check out our spell ebook. There are even more suggestions for the best days to cast spells, along with just about anything else you might need to know.