What's the best day of the week to do important things according to the planets?

It really depends on which astrology you follow, because each planet has an associated day and supposedly influences things differently. Monday is Mars (energy and action), Tuesday is also Mars in some traditions, Wednesday is Mercury (communication), Thursday is Venus (relationships), Friday is also Venus, Saturday is Saturn (structure and responsibility) and Sunday is the Sun (creativity and personal power). The thing is: what do you consider "important"? If it's a business meeting or something that needs clear communication, a lot of people point to Wednesday. If it's something involving relationships or softer decisions, Thursday sounds better. If you need a lot of energy and action, Monday works.

Now, the reality is that this varies a lot depending on which astrological system you consult - some people swear by the moon phases, some look at your entire natal chart, some only care about this when things are really going badly and they need a psychological boost. A lot of people who believe in astrology also say that what matters most is your own chart, not just the generic day of the week. So the honest answer is: if you believe in this, it might be worth consulting your own personalized astrological chart instead of just relying on the day of the week. If you don't believe in it, any day works equally well as long as you prepare properly.