There is symmetry between days 1-3 and days 4-6 of creation week.
Genesis 1:1 is the initialization of space/time and matter. Genesis 1:2 describes the initial state of the cosmos as without form and void. In Hebrew this is tohu va-bohu (תֹ֙הוּ֙ וָבֹ֔הוּ). Tohu means “lacking form and purpose, unordered, desolate”, Bohu means “empty, uninhabited, wasteland”. The first 3 Days were the solution to tohu by bringing organization to the unorganized and the 2nd set of 3 days were solution to bohu filling the organized spaces with inhabitants.
On Day 1, God created light and dark initiating time, and on Day 4 he created celestial bodies (sun, moon, stars) to manage time. Day 2 he separated the water and sky, while in Day 5 he filled the water with fish and the sky with birds. On Day 3 he created the dry land, and on day 6, he filled the land with land creatures. Notice that, Day 3 and Day 6 have bonus creations. On day 3 vegetation and food grow on the land, and on day, 6 God creates man from the dust of the earth like the animals, but made them in his image by filling them with his spirit.
Day 1- Light in the darkness initiating the day and night cycle.
Day 2 – The waters above (the sky) and the waters below (the seas).
Day 3 – The land comes up from the waters, and plants come out of the land.
Day 4 – The “rulers” of the day (sun) and night (moon & stars) were made to mark the seasons, days, and years. These “rulers” fill the heavens for the cycle of night and day created on day 1.
Day 5 – The flying creatures fill the skies above and the sea creatures swim in the waters below, filling the spaces divided by the firmament on day 2.
Day 6 – The rest of the animals are bound to the land including humans. The humans are made in the image of God and rule all of the other creatures to help fill the land.
To the ancient world the sun, moon, and stars were seen as living creatures not just balls of gas or rock. That is why pagan cultures worshipped them as gods. To the Israelites, they were just members of the host of heaven who serve God directly, like angels, cherubim, seraphim, divine council, etc. According to this understanding, they were created to represent time and fill the light and darkness created on Day 1. This explains why our planets are named after Roman gods, as well as the first six months of the year and even the days of the week in English and other western languages are named after the gods of Europe.