Sunday, is according to the [wikipop]Chaldean Order[/wikipop], ruled by the [wikipop]Sun[/wikipop]. That means the Sun has glory and strength on Sundays. Perhaps they meant more that it was a sacred day for the Sun, meaning that ceremonies to honor the Sun were done on this day. We can do just such. It is a natural yogic, or taoist, or naturalist, or spiritualist activity to meditate on the nature of the energy of each day through the week. They do have the qualities of the astrological body/ being that rules them, and each is ready to help you in its activities, if you, “tune in” to it, so to speak.
The Sun has a breathing cycle, it breaths in and out exactly 9 times a day in a rhythmic cycle. During this cycle its surface expands and contracts about 8 miles up and down. In the west we have lost the notion that everything is alive, that all form has spirit, that all form is occupied by being. This is what [wikipop]Shinto[/wikipop]offers as a teaching to humanity. If we look at the Sun from this perspective, and understand how without the Sun, life on Earth, our life could not happen, then perhaps it makes sense to honor, and respect the Sun once a week.
In your chart the expresses itself as your noble goodness. When you pay respect to the Spirit of the Sun, that is what you are honoring, your all virtuous noble goodness. The sign it is in is your Archetypal goodness. Lets say you are an Aries, then your Goodness is enthusiastic initiating energy! Don’t let anyone dismiss it!. Or perhaps you are an Aquarius, then conservative innovation is your goodness! go with it. Your Sun sign is how you shed light on the Earth!
The house the Sun is in is the display of that Goodness, thus in the 8th it is deep mystic goodness concerned with understanding the truth of birth, death, and the struggle of the soul!
Contemplate this, while you enjoy your SunDay!