On the Allegories of the Sacred Laws (1.1-1.5)

And on the sixth day God finished his work which he had made. It would be a sign of great simplicity to think that the world was created in six days, or indeed at all in time; because all time is only the space of days and nights, and these things the motion of the sun as he passes over the earth and under the earth does necessarily make. But the sun is a portion of heaven, so that one must confess that time

When, therefore, Moses says, God completed his works on the sixth day, we must understand that he is speaking not of a number of days, but that he takes six as a perfect number. Since it is the first number which is equal in its parts, in the half, and the third and sixth parts, and since it is produced by the multiplication of two unequal factors, two and three. And the numbers two and three exceed the incorporeality which exists in the unit; because the number two is an image of matter being divided into two parts and dissected like matter. And the number three is an image of a solid body, because a solid can be divided according to a threefold division.

Not but what it is also akin to the motions of organic animals. For an organic body is naturally capable of motion in six directions, forward, backwards, upwards, downwards, to the right, and to the left. And at all events he desires to show that the races of mortal, and also of all the immortal beings, exist according to their appropriate numbers; measuring mortal beings, as I have said, by the number six, and the blessed and immortal beings by the number seven.

First, therefore, having desisted from the creation of mortal creatures on the seventh day, he began the formation of other and more divine beings.