There are many things you can consider when planning to do spell work. Colors, candles, herbs, crystals and also days of the week!
Each day of the week is ruled by a planet which gives it a unique energy. You can tap into these energies when you want an extra boost to the energy of your spell. Along with the planets come elements, colors, deities, and other correspondences. By knowing the energy of the days of the week you can also pair spell components with similar qualities to add even more of a boost – things like herbs, crystals, and incense.
If you end up performing a spell on the “wrong” day of the week, that’s okay! Doing spell work on a day of the week that doesn’t line up with its intent will not have any negative effects on the working. The days of the week will only add energy if timed right, they’ll never take anything away.