Once, in the ancient times, the week has been established. Some nations had 10-day week, some 20-day week, some 5-day week. We have 7-day week. Is our week 'better'? No, but it's most dominating one, just like the rest of the 'western' called culture.

(BTW, 'western' in the ancient philosophy means 'decending'; 'eastern' on the other hand means 'ascending' - I believe it is not just a flat terminology - it is an actual symbolism of the current times. In other words, our current civilization is about to... decend, if to be delicate).

Once it was said that the world was created in the day 1st, which appeared to be sunday. Thus sunday became the first day of a week. And the solar system (our world) is believed to start with the emergence of the sun (Ra); then the rest followed. The cycle ends with the saturday, the day of the darkness (Seth).

Because sunday was set the first of the week (we know it from Babylonians), it may be considered the most powerful, just like the sun is the most powerful object in the solar system. It is very powerful to use this day for magic beginnings, especially at the spring equinox (it's now ).

So, why now the sunday is the last day of the week, following the saturday?

Let me skip the official story, and put it into the practical context. The energy of sunday is very different from all the other days in the week: the society is under the propaganda of what they should do in the Saturday evenings - drink and exploit their bodies to the late hours, so that they sleep in sundays. This leaves the Earth for this short time to be very clear energetically, and calm place. No chaotic brains producing lots of energetic "garbage".
And, hence, this is the most powerful time for the magic.

Take advantage of this situation, and don't leave it to the so-called "dark lords" only, those who have a business in control and enslavement of humans.
You will encounter much less energetic obstacles in the way for your endeavors than in any other day of the week. Most of visions and creation for the world of what is could become should be empowered while most of the humanity is sleeping.