Thank you for your question. It is simple yet very profound. You ask about the first day so I assume you are referring to the reference to the first day account of creation as told in the first chapter of Genesis.

Gen 1:1 In the beginning God created the heavens and the earth. Gen 1:2 And the earth was waste and void; and darkness was upon the face of the deep: and the Spirit of God moved upon the face of the waters. Gen 1:3 And God said, Let there be light: and there was light. Gen 1:4 And God saw the light, that it was good: and God divided the light from the darkness. Gen 1:5 And God called the light Day, and the darkness he called Night. And there was evening and there was morning, one day.

We do not know from this passage if there was light already emanating in the heavens (the universe) after Gen 1:1 and God just let it penetrate the earth’s atmosphere; or if God merely ignited the Sun in our own solar system which illuminated the earth, or if there was no light in all the universe and then he switched it on at that point by the power of his command. The possibilities are interesting.  However, the main point here is that God established a difference between light of day and darkness of night. He established for mankind, in the first day of creation, the basis for “time” as we know it. Time is important because He also set in motion a plan that would ultimately result in a finite number of his created human beings ultimately becoming divine creatures with his very nature and character of wisdom, love, power and justice. This plan had a definite timeline. In order for us to understand this plan as it unfolds, we needed a basis to measure it by. Seconds, minutes, hours, days, weeks ….etc. are all possible because God created light; He separated the evening and the morning into a unit of time called a “day”. God’s time line goes on to detail 6 days of creation and 1 day of rest which are all symbolic of his divine plan.

I would invite you to further investigate this plan. It is one of logic and hope. You can find it at electronically at http://www.biblestudents.com/htdbv5/indexa.htm. This provides you an overview of the Divine Plan of The Ages. From there you can delve into a more thorough explanation of each phase of the plan in plain logical sequence as revealed from God’s own word. There is a free online course you can learn more from at http://www.biblestudents.com/free_online_bible_study.cfm or you can receive a free copy of the overview in printed form at http://www.biblestudents.com/freebook.cfm.