A regular day is defined as having a morning and an evening.
And every day of creation was qualified as having a morning and an evening.
And the evening and the morning were the first day.
And the evening and the morning were the second day.
And the evening and the morning were the third day.
And the evening and the morning were the fourth day.
And the evening and the morning were the fifth day.
And the evening and the morning were the sixth day.
Seriously, it’s a supernatural event. There is no need to appease the atheists by saying a “day” is a billion years.
The book of Genesis was written using a form of Hebrew that is meant to be taken literally. It is a history/law book. It is not apocryphal (sp?) nor symbolic.
The word used for "day" literally means 24 hours. The standard earth day with which we are all famililar.