What days of the week are ruled by what planets?
In many forms of witchcraft and magic, the days of the week are associated with specific planetary influences.
This belief is rooted in astrology. Based on the idea that planetary movements can influence various aspects of life. Making specific days the most optimal time to do specific magic and spellwork.
It's not required that you follow the planetary days for your spellwork. Your spells will work just fine doing them any day you feel called to. However, if there is something very potent you are wanting to work on - using a specified day of the week can help to amplify your intention.