Each day is ruled by a planet. The planet that rules the day is the planet that rules the hour at dawn for that day. Since at least Roman times this system has been used and is now adopted world wide. It is one of the ways we still pay honor to the planets as having influence over humanity and our measurement of cycles and time. The rulers of the day are as follows:
Sunday is ruled by the Sun
Monday is ruled by the Moon
Tuesday is ruled by Mars
Wednesday is ruled by Mercury
Thursday is ruled by Jupiter
Friday is ruled by Venus
Saturday is ruled by Saturn.
Each planet is strengthened during its day. This planet is more active in its natural energy, for better or worse. You can look for the indications of this planet on these days. You will find that it is emphasized in your own natal chart with its indications, as well as in the experiences that arise during the day. A delightful way of coming to observe this is a creative work done on the current day or a journal. Each energy can be invoked in its own way. Since the planets are not theistic, but rather natural energy with intelligent responsiveness, we can apply them to whatever belief system we use. This is why we can find in all the natural spiritual systems, there is compatibility with astrology.
One of the ideas of the great Greek Astrologer [wikipop]Vettius Valens[/wikipop] proposed in has Anthology was the idea that the year iswas ruled by the planet whose day the first day of the year fell upon, same for the month, as well as the day and the hour. It is an interesting idea that deserves exploration. You can look at what day your birthday falls on to see the “Intelligent Creative Energy” ruling your year. Then look in the chart to see how that planet effects your natal chart.