Like other descriptions of time in the Word, "a day" or a number of "days" describe a spiritual state rather than our natural concept of time. A spiritual "day" describes a state of mind or a state in our relationship with the Lord rather than a 12-hour or 24-hour span of natural time. "Day" describes a state in which we are turned toward the Lord, and are receiving light (which is truth) and heat (which is a desire for good) from the Lord.

402. Internal Meaning of Genesis, Chapter 1 (1)

1-31 In the spiritual sense the new creation or the regeneration of the men of the Most Ancient Church is here described; the process of their regeneration from firsts to lasts is here contained in its order.