Scripture does not always give explicit answers for every conceivable question we can imagine - and we can imagine quite a few. Remember, its primary function is to lead us to saving faith in Jesus Messiah. Anything beyond that merely panders to curiosity, and even the mighty Paul admitted he only knew in part. But like him, one day we shall know in full.

Having said that, we can often infer plausible answers from what little info we have, and I believe you have already hinted at the answer, possibly without realizing it. The reason is that important as creation is, God intended His rest to be distinguished in some way from His 'toil' precisely because it prefigured the rest God's ppl would have in the promised Messiah. 

Initially its observance became a legalistic requirement for the Jews, a literal period of 24hrs. But they never understood its deeper meaning, and surrounded it with hundreds of their own petty rules, as they did with practically everything else that was given them. In effect, they turned revealed truth into a religion - Judaism. When they saw Jesus breaking their petty rules, He had to remind them that it was instituted for our benefit, and not the other way round (Mt 12: 1-14 & Mk 2:27).

So, because the 7th day is closely tied to our redemption as a type of what was to come (Heb. 4), which in no way can be confined to a literal day of 24 hrs, one way of distinguishing it from the days of creation would be to describe it in different terms - thus day and evening. But those words are meaningless without a Sun, which, as Sjaak reminds us, did not arrive until the 4th day. 

So, despite the apparent precision with which each day of creation is described, the clues are still there that they do not necessarily represent literal periods of day and night. Do you not think it a little odd that an Omnipotent God would need exactly 6 multiples of 24 hrs to create the very different components that make up our Universe?