That seems like a ridiculous question! Most of us learned when we were very young that a day is a 24 hour period. A day is determined by the amount of time it takes the earth to make one rotation, which requires it to spin approximately 1,000 miles per hour. However, the concept of a day is often questioned when it comes to the Bible, especially during the creation week.
The evolutionary idea of billions of years has infiltrated the church and has led many to believe that the day in Genesis 1 must have been long periods of time. They accept that science must be correct and that the Bible must be figurative or wrong in the creation account in an attempt to harmonize scientific theory with Biblical truth. The problem is that this idea has undermined the faith of many and has caused some to deny outright that the Scriptures are the inerrant word of God.
The Bible gives the account of the first day, “And God called the light day, and the darkness night. And the evening and the morning were the first day.” (Gen 1:5) The very first day was part night and part day. For approximately 3500 years the nation of Israel patterned their day after the Biblical account of evening and then morning. Even now their days begin and end at 6 PM. If this day were a long period of time, (Some suggest thousands of years), then it would have been light for several thousand years and then became dark for several thousand years. Without light, it is impossible for life to exist. Therefore, this idea of a long day is immediately incorrect.
Israel not only patterns their day, but they also model the week after the creation account by resting on the seventh day. God commanded the Israelites, “Six days shalt thou labor, and do all thy work: But the seventh day is the Sabbath of the Lord thy God…For in six days the Lord made the heaven and earth, the sea and all that is in them, and rested the seventh day…” (Ex 20:11,12 KJV) The Lord instructed the people of Israel to follow the pattern of a week that He had established during creation. They were to rest one day, a literal 24 hour period, after working six days just as the Lord had created for six days and rested on the seventh day. If these days were not literal 24 hour periods, but long spans of time, then God was instructing Israel to work for many years without a break and then to observe a Sabbath day that would have lasted for years as well.
In the Old Testament Scriptures, the Hebrew word for day is yom. It is used to represent the daylight part of the day. (Gen 8:22) In rare cases, it is used of an indefinite amount of time. Biblical scholars note that it always refers to a literal day when it accompanied by an ordinal (number). For example, Joshua and the army of Israel marched around the city of Jericho seven days. “But the seventh day you shall march around the city seven times, and the priests shall blow the trumpets.” (Josh 6:4) The number ’seven’ with ’day’ indicated that the passage is referring to a literal day consisting of 24 hours. When you read the creation account, this is exactly how it is recorded; “the evening and the morning were the first day…second day…third day…etc.” (Gen 1:5, 8,13,19,23, and 31 KJV)
The Genesis account records that each day has an evening and a morning. It states that God made two great lights, one to rule the day and one to rule the night. (v. 16) The lights in the heavens would be for signs and seasons and days and years. (v 14) It distinguishes time in a way that has been consistent throughout human history. Therefore, any unbiased reader of the first chapter of Genesis would come to the obvious conclusion that the Bible account refers to a day as a literal twenty-four hour period of time and that the Bible depiction of the creation week is accurate.