source article: http://www.byzant.com/Mystical/Symbols/Septagram.aspx
The septagram is a seven-pointed star, a symbol of synthesis and mystery due to its links with the number seven. Seven is an integrating number, encapsulating as it does the hierarchical orders of classical mystical thought. There are seven planetary spheres governed by the seven planets of classical astrology, seven colors in the rainbow, seven days of the week and seven distinct notes in a diatonic musical scale.
The number three relates to Heaven and the higher realms, not just through the concept of trinity but also as the vertical axis of three-dimensional space; and the number four pertains to the Earth and manifest realms, partly through its relation to the square and the four directions of two-dimensional space. Seven and the septagram can thus be considered as representing the sum of Heaven and Earth, and they allude to the seven directions of space - two directions for each dimension plus the center. As an aside, twelve, the number of signs in the zodiac, is a number of totality, expressing the multiplicative possibilities of Heaven and Earth as well as Heaven in a new mode (12 and 3 are numerologically equivalent, as 1 + 2 = 3).
As an expression of the number seven, and in representing occult intelligence and the beauty of wholeness (rainbows and symphonies are weaved from seven), the septagram is linked to the Kabbalistic sefira Netsach. Other seven-fold systems that can be related to the septagram include the seven latifah (subtle faculties) of Islamic mystical thought and the seven major Hindu chakras - the base chakra (Muladhara), the sacral chakra (Svadisthana), the solar plexus chakra (Manipura), the heart chakra (Anahata), the throat chakra (Vishuddi), the brow chakra (Ajna) and the crown chakra (Sahasrara).
The association of the septagram with the seven planets of classical astrology has already been mentioned, and the septagram above has these planets distributed around its points. Beginning with the point of the Moon, moving counterclockwise around the edge of the septagram gives the order of the corresponding sefirot moving up the Tree of Life (see the astro-kabbalistic hexagram):
Each of the seven days of the week are named after Roman or Norse designations of the seven planets of classical astrology, either directly or through an associated deity. Beginning with the point of the Sun, tracing the septagram clockwise along its lines yields the days of the week in order according to their planetary roots:
Sunday - Sun
Monday - Moon
Tuesday - Tui (the Saxon Mars)
Wednesday - Woden (Mercury)
Thursday - Thor (Jupiter)
Friday - Frygga (Venus)
Saturday - Saturn