Of the Perfection of the Number Six, Which is the First of the Numbers Which is Composed of Its Aliquot Parts
These works are recorded to have been completed in six days (the same day being six times repeated), because six is a perfect number—not because God required a protracted time, as if He could not at once create all things, which then should mark the course of time by the movements proper to them, but because the perfection of the works was signified by the number six. For the number six is the first which is made up of its own parts, i.e., of its sixth, third, and half, which are respectively one, two, and three, and which make a total of six.
In this way of looking at a number, those are said to be its parts which exactly divide it, as a half, a third, a fourth, or a fraction with any denominator, e.g., four is a part of nine, but not therefore an aliquot part; but one is, for it is the ninth part; and three is, for it is the third. Yet these two parts, the ninth and the third, or one and three, are far from making its whole sum of nine. So again, in the number ten, four is a part, yet does not divide it; but one is an aliquot part, for it is a tenth; so it has a fifth, which is two; and a half, which is five.
But these three parts, a tenth, a fifth, and a half, or one, two, and five, added together, do not make ten, but eight. Of the number twelve, again, the parts added together exceed the whole; for it has a twelfth, that is, one; a sixth, or two; a fourth, which is three; a third, which is four; and a half, which is six. But one, two, three, four, and six make up, not twelve, but more, viz., sixteen. So much I have thought fit to state for the sake of illustrating the perfection of the number six, which is, as I said, the first which is exactly made up of its own parts added together; and in this number of days God finished His work. And, therefore, we must not despise the science of numbers, which, in many passages of holy Scripture, is found to be of eminent service to the careful interpreter. Neither has it been without reason numbered among God's praises, “You have ordered all things in number, and measure, and weight.”
Source: City of God (New Advent)