The circumference of a circle is approximately 6 times its radius (Pi times the diameter).

The center of the circle represents Shabbos which is surrounded by the six days of the week. In Kabbalistic terms, Shabbos is the Sefirah (attribute) of Kingship, a single point, whereas the emotional attributes extend to all six directions (up, down, right, left, front, back).

Just as the circle is dependent on its center, so are all the days of the week blessed by the Shabbos. Furthermore, the further one travels out along the radius from the center of the circle, the greater the area. So too with Shabbos, the further one is from it, the more numerous the vessels, (opportunities for interaction with the world).

Reshimos of the Rebbe, Vol. 3, (Chanukah)