Note: This article is adapted from an answer I wrote to a question at another site.
There is much confusion regarding the Biblical account of creation, specifically between so-called 'young earth creationists' and others outside that camp, regarding whether the 6 days the book of Genesis says God created everything in were literal 24-hour days. To be clear, the Bible leaves no room for doubt that God completed His work of creation in 6 literal days. The problem among those who believe the Word of God is inerrant is whether those "days" are meant to be longer periods of time than what we consider a day to be.
In the original Hebrew of the Old Testament, the word translated as "day" is "יוֹם" (transliterated "yom"). However, this word is used to denote an unspecific amount of time, ranging from just the daylight hours, as can be seen in Genesis 1:5:
"And God called the light day, and the darkness he called night..."-Genesis 1:5a (NKJV)
to one "day cycle", as in the very next part of that same verse:
"...And the evening and the morning were the first day."-Genesis 1:5b (NKJV)
to also being used to refer to the occurrence of important events, as in "the Day of the Lord" in Isaiah 2:12.
A little known fact that is often left out of this type of discussion is that we ourselves do not have 24-hour days.
In the realm of science, there are two specific definitions of "day" we can use: the "rotation period" or "sidereal day" which is defined as "one full rotation of the Earth around its axis", and "solar day" which is defined as "the time it takes for the sun to go back to its position in the sky". These two are not equal.
For an excellent explanation of this see the article "How long is a day?" found at maths.org.
Here are some relevant quotes from that source:
"It actually takes the Earth slightly over 23 hours and 56 minutes to rotate once around this axis. In this time all the stars appear to revolve once around the Earth and return to their starting positions. Astronomers call this period of time a sidereal day."
"But the Earth's axis is inclined at an angle of about 23.5° from the perpendicular to the plane of the Earth's orbit. And although the Earth's orbit is close to being circular, it is actually an ellipse, so it is shaped like a slightly squashed circle."
"These two factors mean that the time taken for the Sun to return to a position due south in the sky varies slightly throughout the year. On average this period of time is 24 hours. But at some times of the year it is slightly longer and at other times of the year it is slightly shorter."
In other words, it is eisegetical to force the meaning of "24 hours" for the word "day" (in either language), since even in our present time, we say a day is 24 hours only out of convention and pragmatism, not any other reason. (Imagine the mess it would create if we started using strict sidereal days, or worse, solar days, which would require keeping up with how long each day is... as it changes every day.)
So, how long is a day? We cannot say that a day is by necessity 24 hours, therefore, we also cannot say that a yom is 24 hours either. Especially since the sun (or "greater light to rule the day") was not created until the fourth day (Genesis 1:16), so there would be no way to determine how long it took the sun to "return to its position in the sky" when there was no sun.
Translating the word "yom" in Genesis 1 to any other word (some have suggested the use of the word "eon" in this passage of Genesis, for example) would add unnecessary interpretation and, especially, greater confusion depending on how people understand that newly selected word. Not to mention the fact that the creation model of 6 yomim with a 7th yom for rest was what the Sabbath Commandment in the Mosaic Law was based upon. Any other word would render the concept significantly troubling.
"For in six days the LORD made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the LORD blessed the sabbath day and made it holy."-Exodus 20:11 (NASB)
So it is important to keep the translation coherent with this reasoning, hence the choice of "day" for other languages too.
In truth, the controversy over "how long is a day" is actually only a century or two old, when people started believing in evolution and saying that the earth was millions of years old (and now billions). This obviously conflicted with the biblical account of how long life has been on Earth (notice I didn't say "of how old Earth is"), and the polemic evolved (pardon the pun) as people chose sides in the matter. Only then did some find it necessary to find a different meaning for the Hebrew word "yom", to attempt to reconcile wanting to sit on both sides of the fence.
So yes, the creation days were literal days, but not (necessarily) 24 hour days.
(An explanation of why a creation day cannot be thousands or millions of years will be laid out in a future article. Stay tuned.)