The Hebrew word translated "day" can refer to a 24 hour day, a period of time, or be translated idiomatically as "when." Which meaning is intended for a particular passage is determined by the context of the word in the passage and comparing usage with other Scriptures. Here are 10 Reasons “Day” means a 24-hour day in Gen 1.
1) The mention of “evening and morning” points to a 24-hour day.
2) The lights given for signs and seasons and days and years (Gen 1:14) indicate that the normal system for time is in place.
3) Gen 2:2 (as well as the 10 Commandments) declares the Sabbath (the seventh day of the week) as holy based on the days and week of creation.
4) “Day” in Gen 2:4 means “when” based on the immediate context. It is important to note that Gen 2:4 is in a different discourse unit than Gen 1:1-2:3. See also how this translation fits comparison with other Scriptures below.
5) Some do not think that Adam could have named the animals in one day (which he did on day six). It may be pointed out that Adam was perfect, with no degeneration due to sin. It also seems that he did not have to name the sea creatures. In the end, God could certainly have empowered Adam to do whatever he desired him to do.
6) Every other time “day” appears with a number in the OT, it means 24-hour day.
7) Everywhere else in the OT that “evening” or “morning” appear with “day,” or “evening” and “morning” appear together, it refers to a 24-hour day.
8) Jesus said humans were made “in the beginning” (Matt 19:14; Mark 10:6).
9) If not literal days (and thus some long periods of time), then death would have come before sin (Rom 5:12; 1 Cor 15:21).
10) Throughout Jewish and Church history, God’s people have understood these to be 24-hour days. It is not until the last couple hundred years that it has been questioned by many scholars.
If we believe in God's "eternal power and divine nature" (Rom 1:20), then it is certainly not difficult to believe that God could have created the world in six day. And since this is the obvious, natural reading of the text, then the only reason it would be questioned is in order to make Scripture fit apparently contradictory scientific conclusions.