I'd be interested in knowing where in the bible it says that the first 5 days were actually a day long? Because the Bible also says a day is like a thousand years...
In the 2 verses that say a day is like a thousand years to God, define the word "day".
As for your question, consider...
1. The very concept and creation of "the day" is from God, who created it solely in relation to the earth.
2. God defined "day" as a single light/dark earth cycle - or the light portion of that cycle.
3. Any time the word "day" is modified with a numerical value (the first day, day seven, forty days, etc) it is always and without exception referring to a literal day.
4. Any time the word "day" is modified with evening and/or morning, it is always and without exception a literal day.
5. While the word "day" is often used idiomatically to refer to an unspecified general period of time, the plural word "days" ("in six days I created...") always and without exception refers to literal days.
6. God clearly and intentionally
equated the six days of creation with the six days the Israelites were to work before taking a day of rest (you work for six and rest on the seventh BECAUSE I worked for six and rested on the seventh).
7. There is ZERO logical or scriptural reason to understand the six days of creation as anything other than literal days - nor even an inkling of scriptural evidence that anyone in the Bible every considered them as anything other than literal days. For example, "He made them male and female from
the beginning of creation" - as opposed to "4.4 billion years
after the beginning of creation".
But, can I blow our collective minds for a second?
The Jews don't believe a day starts until Sunset. For the book of Genesis to say "There was evening, and there was morning- the first day" If there is a Morning after an Evening, and this is The First Day, then technically God Created the waters and the Sky on Day 1, and Light on Day 0.
I believe the Hebrew concept of a day going from evening until the following evening is based on that very first day - which began as evening (the darkness which God named "night"), then had a morning (the creation of light), and then ended at evening again - which also began the second day.
So the creation of light (and separation of light from darkness) was the impetus for the first of all subsequent days - and defined a "day" as a period of darkness, followed by a period of lightness, followed by a period of approaching darkness, which begins the next "day".