We presume you are asking about the length of the days of the creation story in Genesis. Most likely, ‘day’ refers to a long period of time. Keep in mind that scholars identify the creation account in the first chapter of Genesis as poetry, not necessarily a scientific textbook. The Genesis account of creation states that the Sun and Moon were not created until the fourth day (Genesis 1:14) so the traditional idea of a 24-hour night and day is not likely.
There are sincere Bible-believing Christians today, young-earth creationists, who take the Genesis account of creation as literal 24 hour days. There is another category of believers, old earth creationists, who take the Genesis account as ‘long periods of time’ based on the Hebrew language in which the word for day can have multiple meanings. ‘YOM’ the word translated as ‘DAY’ has four definitions one of which is, a very long period of time.
Science discoveries in the past century, from which the ‘Big Bang’ theory was postulated, strongly suggest that creation took place over a long period of time, perhaps as much as 14.8 billion years. Geological evidence for the earth posits an age of 4.8 billion years. God’s creation is truly more amazing than we can ever fathom. Recent scientific discoveries suggest this there are over 100 variables that must be perfectly aligned for there to be any life forms at all on planet Earth.
As mentioned at the outset, chapter 1 of Genesis is truth told in poetry. In Bible poems mountains can ‘sing’ and trees can ‘clap their hands’ to the glory of the Lord. (Isaiah 55:12) In ancient Hebrew there is often a rhyme in rhythm to this chapter in Genesis. We are also reminded in the New Testament in 2 Peter 3:8, “With the Lord a day is like a thousand years, and a thousand years are like a day.” This is why the mystery of creation intrigues us so much as we are limited by time and space but God is not.
Finally, this does not mean that God could not create the universe in seven 24 hour days if He chose. God could have done it in 24 seconds. The evidence of science today suggests a longer period of time for creation. The truth to remember is that God is almighty and the creator of the heavens and the earth out of nothing. He literally spoke it into being. Where there is a beginning there must be a beginner. That person is God!
We like to know with certainty about creation, but far more important is knowing the end of the story. How will you spend eternity? Does the creator of all have plans for you? If you are not sure, we encourage you to read the answer to the Question: How does one become a Christian?