Hebrew for "day"
The Hebrew word for "day" in Genesis 1 is "yome" (Strong's 03117). It can mean a 24-hour day or the daylight portion of it (day as distinct from the night).
Without exception, in the Hebrew Old Testament, the word "yome" is never used to refer to a long period of time, as in thousands or millions of years.
In Hebrew, should the word "yome" be used in an indefinite sense, it will be clearly indicated by the context that the literal meaning is not intended.
First-time use not symbolic
Some people say that the word "day" in Genesis is used symbolically.
This is impossible as a word cannot be symbolic the first time it is used. It can only be used symbolically if it first has a literal meaning.
For example, we are told that Jesus is the "bread of life". We know what this means because we understand the literal meaning of "bread", and are able to apply it symbolically to Jesus. The word "bread" cannot be used in this sense unless it first has a literal meaning.
Likewise, the word "day" cannot be used symbolically the first time it appears in Genesis, as this is where God introduced the word "day" and defined it as He created it.
Some might argue that this point is flawed because Job is an earlier book, in the sense that Job lived before the time of Moses. But this is to imply that the Holy Spirit was outdated when He inspired Moses to write Genesis, and that He made a mistake when He put Genesis as the first book of the Bible.
The Bible itself defines "day"
Many Christians forget that the Holy Spirit himself has defined the word "day" the first time it appears in the Bible. A basic rule of thumb in Bible study is to let the Bible interpret the Bible.
* Genesis 1:5
5 God called the light "day," and the darkness he called "night." And there was evening, and there was morning-the first day.
The first time the word "day" is used, it is defined as "the light" to distinguish it from "the darkness" called "night".
The phrase, "and there was evening, and there was morning", is used for each of the other five days of creation. This shows that there was a clearly established cycle of days and nights (periods of light and periods of darkness).
Incidentally, those who argue that the word "day" in the above verse means millions of years must answer the question, "What is a night?"