It is sufficient to enumerate six Days, in which number, variety, and adornment were to be completed. Thus on the First Day spiritual creatures were formed; on the Second, the superior corporeal ones; on the Third, the inferior corporeal; or, according to others, three Days were given to variety, and three to adornment. And so the Divine perfections correspond to the perfections of these six numbers, viz. one to the formation of spiritual creatures, two to that of corporeal creatures, and three to adornment.
Nor does the Author intend to contradict the opinion of Augustine that all these Days are but One Day, which presents things after a sevenfold manner; for by day he understands the apprehension of the angelic mind; so by the First Day the first work, by the Second Day the second work, and so on; all that God produced in the order of nature being successively impressed on the minds of the angels, which can apprehend many things at once, principally in the Word, in Whom their knowledge is perfected; and these are called days because light, which is the cause of day, belongs chiefly to spiritual things.
Others, indeed, see in these Days a succession of temporal days, and things created under their specific forms. Thus there are several differences of interpretation, but Scripture makes use of suitable words for expressing the work of Six Days; and in various modes shows forth the Father as creating, the Son as the principle, and the Holy Ghost as complacency.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni