Let There Be Lights in the Heavens

God created the sun, moon, and stars as light-bearing signs of the days, years, and seasons. These celestial bodies were not created to define the day, but to sign the day. God obviously didn't ordain the day on the fourth one, but the first.

One of the reasons we know the days of creation are the same as the days we experience now is that the when the heavenly bodies were created in accordance with the morning and evening, they did so according to the pattern of evening and morning that God had already established on the first day.